Using the command /execute as @e[type=upgrade_aquatic:thrasher] run say test and counting the lines in the log, there's 48 Thrashers right here. That's so many Thrashers.

This is using upgrade_aquatic-1.16.5-3.1.0

There's also no other ocean life in the surrounding area, besides like 2 Frilled Sharks from Alex's Mobs

Change the rates of Thrashers in your config. Their spawn rates are designed around vanilla cave generation.

I see this issue is closed, but I'm having the same problem. This is after reducing the spawn weight of thrashers in the config from 90 to 10 and loading a completely new deep_frozen_ocean biome:

The same test as op shows 44 thrashers and 16 great thrashers in the area.