I looked around but saw no option. It would be nice if we can change this value because the mobs are often far away or even not directly in reach if you have a big cave underneath.

It is currently hardcoded, I check every mob that spawns, so the 128 blocks is given by Minecraft’s spawning mechanics

I played around with making it configurable but didn’t do it in the end for 2 reasons:

1. it introduces a little more lag, though it won’t really be noticeable on its own
2. I don’t think that it solves the problem of MobCaptains spawning where you can’t reach them

Therefore I'd rather look for a different solution like reducing cave spawns and buffing surface spawns or something along those lines

Yes, please, that would be nice! 🎉

Oh yes, this makes very much sense. Your solution sounds good. When I can test it or can help you, just say it.
I use a modded minecraft server via fabric loader and version 1.20.2 😊

Greetings LxWulf ✌️

I can let you know here, when I found the time to implement that feature 🎉