I'm not saying that this is a good or bad idea, but I think something this extreme would be the only thing that would radically slow down the speed of a wipe.
You lock players getting past a certain level, for a certain amount of time. For example: 1st week of the wipe: everyone is capped at level 10, 2nd week capped at 20, etc. What levels and how long the level caps there for are can be played around with, but the idea is that that first day/week that many players say they enjoy, when everyone has shit gear and meds and will use anything they can get, would last longer.
"But what if I want to really grind and get cool gear before others" I hear you ask, well you still have scav bosses and raiders for this, meaning you can still get that gear, it's just far harder.
"Okay, but it's not in the spirit of Tarkov to be forced to (almost) not be able to meta M4 someone only wearing PACA" I hear you say, and I understand this, but I think it just might be worth the sacrifice.
Lastly, I don't see how you could ever increase the length of the extremely early wipe chaos without something drastic like this. Understandable if you don't want that, but I think it'd be sick.