All that said, if they can redesign Pact Magic in way that doesn't recharge on a short rest (without changing the 1-hour short rest itself which I like), that also lets Warlocks keep a bit of big bang of higher level spells at proper spell levels, that is worth exploring too. Below is one suggestion I have that lets Warlocks get the same power-level of spells at the appropriate spell levels:
- Warlocks could choose between Cha, Int, and Wis for their spellcasting ability modifier. That does not define the Pact Boon they choose at 1st level. Want to be an Int-based Pact of the Blade? That's fine. (Is Con-based spellcasting too powerful as a SAD option?)
- Warlock gets 1 Pact Spell slot for each spell level at the level that a primary spellcaster would get that spell level (A 1st-level slot at 1st level, a 2nd-level slot at 3rd, all the way up to a 9th level slot at 17th). The Warlock chooses the spell they like, and it stays that way, however they can "prepare" different pact spells by communing with their Pact Object (blade, tome, or familiar) at the end of a long rest. Those spell slots recharge on a long rest (there is no short rest mechanic in this system). This means they would get up to 9 "daily" spells from this ability at 17th level (more from Patron spells later). Because of the number of pact spells they will get (even more from Patron Spells), these do not auto-upcast, and can only be upcast if they are prepared into the appropriate spell slot ahead of time (you could prepare multiple fireballs in higher level slots).
- Due to the change in power level, Mystic Arcanum might no longer grant new spells in this paradigm, as it is effectively rolled into Pact Magic. But if balance determines it can add 1 extra spell slot for 6th through 9th for the cost of a Mystic Arcanum, that could be fine (though I doubt that having access to two 8th and 9th level spell slots per day would fly).
- At 3rd level, Patron Spells add new Pact Spells, adding 1 more daily spell slot for each spell level (expanding up to 5th level) for specific spells on a list (these Patron Spells can also be used in place of normally prepared Pact Spells of the same level). Maybe as another ability at 3rd level, in addition to the Patron Subclass, they get a lesser "Commune With Patron" ability that allows the Warlock to commune for an "Arcane Recovery-like effect" to get some spent spell levels back, as a nod to the "short rest" functionality. It also reinforces their connection to the Pact holder for acquiring power.
In this rough design, Warlocks don't get as many spells overall compared to a full caster, but they ultimately get 2 spells of each spell level up to 5th (with a few more spell levels from Arcane Recovery), and they still get up to 9th level spells, and they still get their Invocations (which might be abilities, short rest, or long rest spells), and still get their Pact Cantrips. The Warlock's power level access would be on par with full spellcasters at the same levels.
With this design, an 11th level warlock would have 11 daily spells just from Pact Magic (2 spells each for spell levels 1-5, and a 6th level spell). They also get to recharge up to 5 levels of spells on a short rest, once per long rest. They also have 6 Eldritch Invocations to tweak their Warlock's theme.