I normally get all the insurance I can get but have taken a few holidays without covering Prostate Cancer. Although I'm currently undetectable after the op so a dramatic event seems unlikely.
That said I still try to book fairly close to the holiday and look for holidays with the ability to change dates, or pay the full amount as late as possible. Losing a deposit can be less than the insurance especially if you pay a supplement for better cover.
This might not be helpful but possibly if I was Stage 4 and wanted a holiday I'd try to find out the worst that could happen medically and if I could have a scan or something to see if it's likely to happen. If you went a few years with nothing happening you might kick yourself for not going. On the other hand if you needed urgent care and an air ambulance it could cost a lot of money. I guess you've been through all that.
I'd also be careful about buying an insurance that doesn't actually do what the headline says it does. I'd read the small print.
I hope you find something.
All the best, Peter