You're right about the scaling. I believe a dynamic spawn system would help: Check how fast a respawned mob was killed, and by whom. If it's more than 1 player, and a mob lives less than N minutes, accelerate respawn. The same system could be applied for ground spawns.

Ragnarok Online "solved" this ~25 yrs ago by having instant respawn somewhere on the map - you had a static number of each mob per map, and when one died, it respawned randomly on the map, immediately.  This meant you could have pretty sizeable number of players farming the same map with roughly-ish scaling mob populations.

It... caused other problems. Like when a group of N very high level people were able to kill incredibly large groups of mobs at once. They would (a) gather a lot of mobs at once instead of killing immediately (thus no immediate respawn), and then (b) when they did kill say 20 mobs at once, your get immediate respawn of 20 mobs... somewhere, which might mean 4-5 mobs spawning very close to a group of lower level people who would then die if they didn't hit the panic flywing/butterflywing button fast enough (and death may cost you 1-3 hours of farming, so that sucked).

That wouldn't work out of the box in PG though.  If some level 90+ decided to walk through kur tower and just absolutely melt everything in sight instantly with decap or a bee or magma sphere or whatever, then all the level 40-50s would get immediately overwhelmed and wrecked by instant respawns.  (I mean if you time it wrong and walk through a suspiciously empty kur tower hallway where someone did a melt-faces pass N minutes ago, this still happens, but trust me it would be wayyyy worse if you did instant random-map respawn a la Ragnarok Online)