Rather than the Sorcerer class, how about reflavouring the Monk class? Use Magic Initiate to get some cantrips if you don't want to take a level of Sorcerer.
Damage auras can be used to warn PCs that this is a serious challenge, maybe one beyond them. For instance if you have the critter wade through mooks killing them all it gives the PCs a round or two to prepare - or flee.
It's different. Both the spear and the javelin have the thrown property; sling bullets and arrows do not. (Actually, I think the bow and sling should key off Str, not Dex, but that's for another thread.)
(I don't have a Beyond account so cannot see the stats.)
Why are you assuming magic equipment? CRs are normalised against a party without magical equipment, are they not? If you're assuming equipment that bypasses an opponent's defences then of course the effective CR of that opponent is going...
No, because you're swapping a defensive ability for an offensive one. Plus the player is already swapping it out for the ability to do more damage by using a two-handed weapon or two weapons.
I'd rather give all Long Rest classes recoveries on Short Rests with extended recovery on a Long Rest. Basing everything around Short Rest helps prevent the 5 minute workday.
Not exactly what you're after but in a previous edition there was Mordenkainen's Lucubration which allowed the recovery of one spell of lower level. IIRC it was a 9th level spell. Pushing that into 5E you might make it a 9th level spell that recovers 1 slot of 6th level or lower (depending upon...