Ruin Explorer
Legend
?!?!???!?!?!?!?and especially if those encounters go on to ten rounds or more.
10 rounds?
What the hell kind of 5E encounter goes on 10 rounds?
Talk about burying the lede. If you have encounters going 10 rounds on the regular, that's your main balancing issue, not 1-2 encounters/day.
5E's encounters typically last 3-4 rounds. You have totally and completely ignore 5E's encounter design guidance to get over like 5-6 unless something bizarre is happening. To get to 10? You basically have to be running something totally wild. Which is fine, but that's what the main issue is - you're running encounters that last 2.5x as long.
I dunno why anyone hasn't suggested the easiest possible balance solution though.
Just change Warlocks to Long Rests, and increase their resources on the basis that they'd get 2 Short Rests per day. So
L1 = 3 spell slots/day.
L2 to L10 = 6 spell slots/day.
L11 to L16 = 9 spell slots/day.
L17 to L20 = 12 spell slots/day.