Best DSLR lens for whale watching? And/or switching to mirrorless?

Hello, I’ve been in the market for a new setup for a while but I just picked up a new lens for a great deal off Marketplace (Sigma 150-600 F5-6.3 Contemporary) and since I saved so much money there, I’m now considering a body upgrade.

I currently shoot with a Canon Rebel T5, which has some issues, notably a few marks on the sensor which appear in every photo. Most of my photography is whale watching from shore, or sometimes on a boat.

The DSLRs I’ve been considering are a 6D Mark II or a 90D. I also have read in many places that wildlife photography in general is just better done with a mirrorless, but my biggest hangup with this is budget. My goal is to spend less than $1000 on a body. I’m buying used, of course, but hopefully with at least a “condition” or better.

Advice and recommendations welcome!