If you have the time and energy, I'd recommend adding 1 spawn, test, saveworld, restart... and continue until you recreate the crash. Also writing down settings, added spawns and possibly spawn locations. While I can't guarantee this will help darklore in debugging, it is easier to recreate a crash when knowing what was added to a server that did crash.

We have 35 spawners now, 4 nests, spawning in 45 different vanilla/base-game creatures. Several of these are set to spawn 20-50 creatures with a max of 100 in radius (which was the max setting on spawner).

I have written down spawn locations, settings, creatures, etc, so if I were to add a new spawn today, and the server would crash tomorrow m orning, I can assume that newly added spawn is the culprit. Then I can test the same spawn to see if I can recreate the crash...

Make sure to always do frequent backups of your saves.
I have my servers' save files back up every night, and saved for 14 days.
Then they get backed up once a week and those weekly backups are saved for 5 weeks.
I'd rather not have to roll back a server by 5 weeks but I have the option to if something goes wrong.

crashes my game