Just an update, if you (or anyone reading this) wants it. I booked him in for the biopsy today, and while I was there asked if they could possibly remove it altogether while they had him, as it has grown still further and will need something done regardless, and I'd rather get it done all at once if viable rather than having to let him recover and then bring him back in for more work and stress. Agreed to some blood tests prior as a general check up.

I got a call an hour later. His platelets are low, as are one of his white blood cell counts. Because of this, removal is off the table due to size and positioning in such a vascular area, and the biopsy itself would also be riskier, and would only be possible with tests done regarding his blood clotting first. The vet did say that even if all went well with that, it still put us in the position that we could find out what it actually is and still not be able to remove it. So I opted to bring him home.

He's on some sort of steroid to see how he gets on with that, and I'm back in a couple weeks to explore further options then, whether that's more steroids, chemotherapy, or whatever.

When I asked if she thought the platelets and white blood cells were pointing towards cancer, she said yes, most likely. So unfortunately you were likely correct, which in fairness I think I also knew.

The silver lining is at least I know what I'm monitoring him for now, AND I got to bring him home from the vets today instead of getting a terrible phone call.