If you're using Ember for filler actions I don't even know what to say.
I am not, what are you even talking about? "Here's what one character does when faced with trash combat" does not at all lend to "Yeah this guy uses <character> for filler exclusively". That's not even possible. You're free falling into the abyss of conclusion making.
It's Kingmaker. You're level 10 and you run into kobold alchemists. They don't pose a threat to you in any way. This isn't a fight. It's filler. And filler fights beget filler actions. Everyone in the party auto attacks the kobolds to death, the end. A Witch can cast vomit swarm just for the heck of it or cast a Misfortune Hex but it won't do anything. That's not that kind of fight. People are dead already. And its my personal and completely non min maxy preference that instead of firing a crossbow and missing the caster waves a wand of magic missile and deals useless damage against an useless monster.
The thing about filler is that it can happen very often. It can happen during a dungeon crawl. Sometimes they happen over and over again and spending your spell slots on those is a wager that you won't need them in any of the other 20 more pressing fights that may or may not happen in said dungeon. The reason why Witches are comfy is that they can just hex all day long and be at max performance throught all moderate and hard fights. But if it happens that I'm not playing a Witch I'll get the Wizard a wand or a bunch of scrolls.
I already have infinite Fireballs at that point from my MC Alchy.
Infinite or plentiful?
A much nicer filler action for full casters than casting the slumber hex (that was so great at level 1)
But hexes generally are better than fireballs, and Slumber works just fine beyond level 1. Fireball is only any good if you pick mythic to remove energy resistances and preferably stack DCs (hello +25 reflex swarms though). And mythic for extra spells/level will give you 8-12 fireballs anyway if you want them.
Fireball is just a simple example of what I want to do. It's an accessible wand which according to PFR rules has a base caster level of 5. So when you first turn into a trickster at rank 3 it's casting spells at CL 8. It's a nice filler action. A better light crossbow for the equivalent kobold encounters, that's all I want. Even if it just exists in my mind.
Contrary to Desideratio's beliefs I am not min maxing for a broken character at level 20. All I did was describe how the UMD Rank 2 Trickery works. I mean. If you know me I can barely stand arcane trickster since it delays spell level access by ~one whole level~. I am about the experience from start to finish but also about my own little eccentricities. It isn't about breaking the game for me. It's about what I like to do. And as it turns out if there's one thing I hate doing it is resting. At all. I never rest until the game twists my arm over it. If I feel the devs can't help themselves and will make obligatory dungeons with 50+ dumbass fights I'll just refrain from higher difficulties because I will not rest in a dungeon or in the middle of assaulting the enemy maze. So I played Kingmaker on Challenging, but was pleasantly surprised that Owlcat didn't fuck up dungeon design until the House that shall not be named.
What makes it fun is that I also like to play wizards. So I cast with staying power in mind. I pick spells that can shut down fights by a single or at most two castings, which gives me an emphasis on crowd control. The most fun I had in Kingmaker was when I first assaulted Vordakai's tomb. I'd make liberal use of Grease, Glitterdust, Chains of Light and so on to avoid being damaged at all. I didn't have as many death wards as I ended up needing and got to the Lich with one or two spells left on my Sorcerer and everyone kinda fucked up. It wasn't a hard fight. Vordakai's encounter itself is super disappointing. No prebuffing, no personal guards, nothing. Just his familiar. But the attrition of it all almost made up for it. And since I'm playing a game with vancian casting, the uniqueness of the experience pays for itself.