abirdcall
(she/her)
You are grossly overstating the power of a warlock. No matter how many spells they know, they are still severely limited by the number of spell slots. Invocations are great, but the ones worth a damn cost spell slots, and are usually once per day, so its not that absurd.
Patron powers? Everyone has their archtype powers. Same goes for Pact powers.
The fact is, warlocks are far from being in the overpowered tier. They don't even scale well into high level, with Mystic Arcanum being so limited and all. You are over exaggerating.
As written they aren't ovepowered but with your houserule they are.
You are telling me that an average of 9 3rd level spell slots per day at 5th level along with their other powers/abilities is balanced?
It's not even close. Their raw power level is fine. If you want to make the argument that they could have some more spells known or a bigger (but not more powerful) list of invocations to choose from then sure.
But increasing their spell slots by 50%? That's absurd.