So... yeah I don't think using the D&D One Playtest 5 version of Warlocks as the spellcasting chassis is a good idea. Sure, it keeps everyone 'close to 1st level Wizard' for spells at level 1... but with Mystic Arcanum as an invocation...
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You're looking at a structure where any caster doesn't get 5th level spells until 17, and only once a day, unless they take Mystic Arcanum at 9th level and then they can cast a 5th level spell once per day while everyone else is only able to pull 3rd level spells.
That said... I do think the Warlock Chassis is a good basis for doing half-casting core casters... I just think the OneD&D would-be version isn't great. Good for an actual half-caster, though!
Core casters should go to 5th level at the same rate as a 5e warlock, and have as many spell slots at that level as the above half-caster table shows. That way they reach their capstone 5th level spells pretty early in the game and are 'as strong' at spellcasting as a 20th level actual hybrid class by 10th level.
And then do both the traditional Mystic Arcanum granted at 11th, 13th, 15th, and 17th for a single-use of the higher level spells per day, and only one or two known of each of the 6/7/8/9th level spells...
But also short-rest recovery for the 5th level and lower spell slots. Not -all- of them, but some, for the "Full Casters". Something like:
This way everyone benefits from short rests to a significant degree, it puts a limit on how many short rest recharges you can get as a full caster, and it encourages -some- restraint in spell slot use without actively limiting it because you could get one of your two 5th level spell slots back on a short rest, but if you need two, you're SoL.