There's no reason to over-complicte imo, just let people short rest more, like maybe 4 times.
Whatever the system is those who want to abuse it to always get fully healed and full spell slots will do it. Meanwhile for those that won't 4 short rests will help put warlocks a bit more.
What they should do, however, is tie in some of the narrative stuff they have linked to long rests to short rests.
