At the core, I think that the problem seems to be that they are trying to turn a very accessible, popamole game into a hardcore one and it doesn't work very well because Bethesda's design philosophy of allowing the PC to be amazing at everything just doesn't jive with what Requiem attempts to do.
That's only a problem with (some) questlines.
I rolled a Two-Hander/Light Armor Nord warrior character (basically, a barbarian), and I'm at Level 6 right now, so basically I'm getting raped by everything.
Which really, isn't a problem if the game is well-designed, but the issue is that survival in this game has far more to do with luck combined with lots of trial and error than it does with any sort of skill. Or at least, it appears to be that way after ~8 hours of gameplay.
Gitting gud isn't merely a function of patience. You need to actually git gud.
Light armor basically provides insufficient protection. The upsides are that you can hit hard, hit fast and move quickly. The downside is that you have to keep moving - The light armor isn't for tanking, even if you're building a barb and religiously pumping HPs. The bottom line is that light armour allows you to avoid getting hit while still making some difference in case you do get hit. Which means that if you do get hit regularly you're doing it wrong.
The skill comes from learning from your trial-and-error. Luck is only involved in surviving something dumb and (rarely) in Beth's superb code glitching or gracing you with killmove the moment the enemy *starts* to swing (even if you could easily avoid it). If there is no enemy killmove or glitched controls involved, then the problem is you. Even with enemy killmove involved the problem is usually still you because more often than not it means you got cocky and did something you wouldn't afford to if you were fighting an actual human.
Most of the time I die, it's from an arrow fired off so far that I can't even see the shooter.
Then it means the arrow took its sweet time reaching you so you could have been somewhere else already.
There seems to be little rhyme or reason or anything that I as the player can do to anticipate or plan around my enemies.
So AI not being (easily) predictable is now bad?
But perhaps I'm missing some things and there's some nuance to this mod that I haven't quite gotten just yet. So, Requiem vets; what can I do to ensure my character becomes an ass-kicker?
First thing first - keep moving. You are just a dude - even if strong, fast and resilient dude - with some fur or leather on. You can't afford to be stuck full of sharp sticks tipped with metal. You can't afford being repeatedly hacked and slashed.
When charging archers try not move in straight lines and try to exploit anything to break their line of fire - terrain, architecture, even their own allies.
Also, sprinting is your friend, especially in light armor. Sprinting into your typical archer or wizard with a heavily muscled guy (especially when wearing heavy armor, but if you're strong enogh light will work too) will knock them off balance and onto the ground. An enemy who's trying to get up is an enemy that isn't shooting you, switching to melee, blocking or running away to shoot you moments later. If you can't knock them down run up to them and break their bow with normal attack. Try to get them to shoot and miss first so that you won't get an arrow tangled in your intestines point blank.
Second, practice in melee. Low level Draugr (but try to postpone BFB, maybe try doing companions) are ideal practice dummies, they are slow, meaning it's easy to learn how the melee works with them, and they are tanky meaning no fluke victories.
They are also dumb and reckless so you won't get to learn fighting blocking and bashing enemy from them, but you will learn basics - when to block, when to dodge and when to interrupt by bashing. If you can melee two draugr at once you're probably pretty good.
Bandits are harder but more versatile practice targets. They are much faster, more proactive and more concentrated on their own survival. They will block and interrupt. Learn to not get flanked - even a tincan can get 1hk'd by an unblocked powerattack with as much as lousy iron hammer or battleaxe. Your own powerattacks are good for breaking through blocks, but the blocking party can interrupt them with bash. Use length of your weapon to your advantage. With 2h you can hit them before they can hit you, bashes also only work up close - a shield or pommel has limited reach.
Learn to sidestep - see a guy running at you swinging a mace or hammer for overhead strike? Don't block, even don't interrupt, move aside. Then turn around quickly and split their ass.
Get a follower, possibly one complementing your own tactics. For example a 1h+shield+plate tank will have good staying power and will also provide good flanking protection and allow pincering single attackers and exploiting openings (enemy strikes, your fellow blocks while you cut the attacker down).
Lastly, pick your battles and improvise when in over your head. Sometimes it's a good idea to turn your tail and flee.
Sometimes you can use something that isn't really part of your skillset well enough in the context to turn the tides of battle.
I've been flirting with the idea of investing in Archery myself, so I can take out ranged opponents at a distance. Alternatively, I've been thinking of Sneak as well so that I can at least get close enough to strike before they can get a shot off. Any ideas?
Try them first.
For example if you have space to spare try carrying around a cheap ranged weapon and some ammo. Even unperked it should be helpful to pick off an occasional PITA archer or shoot a wizard about to cast a protective spell or some nuke. You can use it to deliver a poison too.
Sneaking won't help much without perks, but it will allow you to come closer before you're detected - unless you need speed or combat stance right now, it's always a valid option.