Heck, I think fast enemies (and thus mage playthroughs) were tougher in Dks 3 because the AI felt more aggressive and enemies also seem to move faster.
It's also because they nerfed the shit out of magic yet again.
What does magic offer? Spike damage and additional range, generally lock-on range unless you're an extraordinarily dab hand with free aiming or are fighting a boss, which is why it's famously used for boss cheesing. The spike damage isn't even that fantastic if you're going for efficiency while working through an area. In exchange, you run through Fag Points extremely quickly, probably have to split your Estus even despite the closeness of the bonfires, have to stand around like an asshole for a second or two being completely vulnerable when you cast, and spellcasting calls for a minimum of two dedicated stats, and sometimes three, sucking points away from VIT, END, etc. Hybrid? Maybe at SL 100+, or a bit of priestly magic. Otherwise, fuck you.
Meanwhile, you only need one and a half stats to use any melee weapon perfectly well by midgame, so you can spend far more on survivability, equip load, and stamina. Nothing ever breaks for any reason despite the existence of a durability system, so you get infinite attacks instead of running out of Fag Points after killing five enemies. The only downside is that you have to get slightly closer to the enemy. That is literally the only downside, which is why I batted almost every magic-using invader into the stratosphere effortlessly, with one notable exception who had a billion hours in the game to my 42.
That's a nice summary of how they gutted spell casting in 3, and I might add that it is the icing on the cake when one considers this is already a heavily desiccated game that is lacking in content, branching exploration, and replayability compared to the others. They couldn't even be arsed in adding some post-game procedural dungeons like the Chalices from Bloodborne. By screwing up mage builds the way they did they just took away one of the biggest factors contributing to replayability in these games, because said builds were always among the most interesting and the first I would try out after running my melee builds.
Frankly, I was spoiled by Dragon's Dogma. DD isn't really a Souls-like game per se, although it's reminiscent in numerous ways, and it's not as relentlessly difficult as a proper Souls game... but boy, did they handle classes correctly. Instead of one person, you have your main character and two pawns. The dexterous classes climb everything in sight and roll and sprint and clamber all over large monsters and shoot arrows tactically, the dedicated rangers snipe, the beefcake classes block and parry and smash and shout and charge, and the spellcasters hide in the back like little faggots chanting and desperately hoping nothing notices they're there.
There's your standard mage, and a dedicated variant who loses access to some healing and support spells but can cast massively damaging AoEs that have an enormous casting time. There's RITUAL MAGIC, yes, if you have multiple casters in a party, they can combine their powers. And, there are proper and balanced hybrids: magic knight, magic archer, that sort of thing.
Dragon's Dogma allows spellcasters to truly be spellcasters by not treating them as frontline gladiators. That's the downside to Souls being designed for one character, I suppose.
I am a big fan of DD myself, and I think it is probably one of the best, if not the best, implementations of the whole concept of Action RPG in the industry. The reason I had always been skeptical of ARPGs in the past was that they usually failed to provide as many options as the heavily abstracted systems of turn-based games. In TB games you don't need necessarily need to implement the actual physics of the abilities used by each role, and you can get away with just providing some symbolic animations for certain abilities rather than having to, you know, program your engine to simulate climbing, fireballs, freezing, slashing, shooting arrows, etc., all in real time. Dragon's Dogma manages to implement at least three (more if you consider the hybrids) very different play styles into its engine, and they all feel like you are playing a different game (archer feels a bit like playing a shooter, warrior like a hack and slash,etc), and the gameplay for each of these options is smooth and competent by the standards of modern action games, and while the combat obviously does not measure up to the very cream of crop of action games, it ultimately does not have to due to the character building options and variety that the gameplay offers as an RPG, which more than justifies its existence as an alternative to both action games proper and the typical Western ARPGs with crappy combat and few options such as Piranha Bytes games and Witcher.
As you said, it is not really like Dark Souls at all. The latter is more of a straight up action game, while DD is more of an RPG. Twitch skills tend to matter more in DS than in DD. I can't imagine doing level 1 runs in DD. Perhaps it is possible, but the HP difference should be too much to make it enjoyable at any level. I usually clean the ogres from the tunnel in DD at relatively low level - soon as I hit Gran Soren - and that gives me an inkling of how annoying that would be. The character system itself and character-building are also arguably more interesting in DD. Not to mention that Dark Souls's exploration is more like a Metroidvania or a dungeon crawler, while DD goes more for the open overworld thing.
I do agree, however, that Dark Souls could learn quite a bit from DD (and for that matter so could the entire ARPG scene), especially how to make different builds more interesting. I mean assassins and archers are super fun in DD, while they always have struck me as kind of lame in Dark Souls. That said DD could also learn a bit from Dark Souls, one thing that Dark Souls does very well - apart from the depth of the combat mechanics - that I rarely see in DD is designing tactical encounters. Like, for example, in Dark Souls areas are designed with the explicit intent to screw you: you have enemies in plain sight that act as bait, guys hiding in ambush waiting to skewer you, campy fucks shooting and throwing crap at you, sometimes environmental hazards and traps, cramped environments and limited field of view, and all of those factors and more engineered to operate in unison according to the Dungeon Master's sadistic plan to bar you from getting through with your estus filled, if at all. Though to be fair, this works for Dark Souls due to the nature of its exploration as a dungeon crawler, perhaps DD could implement more extensive dungeon crawling. One way could be dotting the world with dungeons a la Morrowind. Either way, both are excellent games.
EDIT: Also, I think there are some downsides to implementing a party system in an action RPG. While I think that the latter is a generally good thing by making customization more extensive(party customization is always better than single character customization, even if one is not given control of the builds of every pawn) and makes DD more faithful to the spirit of old-school RPGs than most action RPGs, I think it is probably the most raw element of the DD formula and still needs some work. In particular, I have in mind the whole mechanic of reviving party members while they are down, which I feel is cheesy. They should either go with permadeath or something more balanced like sending them back to town after getting downed, or reviving them only after the end of combat. Also a bit more party control would be welcome, but I suppose limited squad control is always going to be a limitation of third and first person real time RPGs, and expecting the same kind of control one gets from isometric TB or RTwP would be unrealistic.
Another problem in this connection would be one of balance: either the dev makes party members ineffectual and balances enemies to be beaten mostly by the player character - thus maximizing the skill-based element of the gameplay, or they are made effective at the expense of the skill-based nature of the combat due to the corresponding bump in enemy stats required for them to survive a party, which makes the contributions of the player less relevant. This is one of the reasons the melee combat itself is better in Dark Souls, because the player is mostly expected to rely on his own efforts to beat enemies (leaving aside the cheesy summons) and thus the devs can tune the challenge accordingly. It would be something else if there was more party control which allowed the player to use tactics instead of skill, but one may then wonder why go with the real-time first/third person format to begin with. Much of the fun of managing a party in TB and RTwP comes from managing the tactics of your squad, if there is little possibility of doing that it might well be better to take the DS approach rather than the DD one. Still, I am open to see if Capcom, or anyone else, can take a crack at it and succeed.