I was hoping for something that would actually make Primeval Thule feel like the sword and sorcery subgenre in terms of mechanics rather than just setting fluff. (Crypts & Things' rules for magic are pretty fantastic in this regard, making magic dangerous to the caster and adding in the possibility that the magic user could become corrupt.)
The long and short of it is that I'm not a game designer, and I find my free time to tinker around with house rules and the like is in short supply.I tend to prefer things that have already been play-tested and work pretty much right out of the box.
As much as 5e tries to be for everyone, being that flexible meant punting a lot of fine-tuning to the DM, so you might find it frustrating at times if you really need a plug-and-play sort of experience from your RPG.
One approach is to delete from the rules rather than homebrew or re-jigger them. As has been suggested, while you might not have time to homebrew and balance novel magic rules to get the tone you want, you can simply ban casters as a PC option at low level. When the rare monster or NPC uses magic, it can just arbitrarily do whatever you need it to do. There's no need for consistency.
When a PC finally uncovers some magical secrets and gets to take a level in a caster class, his magic needn't work anything like NPC/monster magic has been.
And, since paladins and rangers don't get at-will spells, I'm not really seeing a problem with including them.
there will be NO cantrips
Personally I have come round to the view that at-will cantrips in 5e makes it very difficult to accommodate the Primeval Thule fluff.
The older versions of DnD with daily spells only suit sword & sorcery better I think.
It think it's a little odd that in trying to get a 'lower magic' feel, one of the first things on the chopping block is the least powerful of PC magics. Cantrips are few, not that flexible (you learn them and that's it, you don't change 'em out each day) and are already designed/balanced to avoid being systematically abuseable.
Spells, OTOH, get very powerful, varied, and flexible, and are designed with the idea that there will always be pressure on the slot resource, preventing systematic use/abuse. The moment you have a slow day or some downtime, though, spells /can/ be cast systematically.
A character who can throw a bolt of fire whenever he likes (and occassionally starts a fire without meaning to) and not much beyond that is a bit more S&S than one that can 'only' cast six powerful, dependable spells a day, every day, and can decide that morning what each of those spells will be.
The versatility, power, and relative dependability and, well, safety, of Vancian magic is much more at odds with S&S than a small number of relatively minor, specific, magical effects, even if they are mechanically 'at will.'
the closest 5e class to what I envision is a Warlock with spell "slots" recharging after short rests -- not Vancian magic
I agree that the Warlock, sinister, and not so Vancian, is the best fit of the caster classes. Eldritch Blast & all, IMHO.