Hello, I have the same cancer type as you and have been on palbociclib, similar to ribo, since December 2020. I travel a couple of times a year and have not had a problem with insurance. I’ve been around Europe, Australia, Singapore and the Maldives. Never a problem. I’ve got annual insurance for me, husband and 2 children and cost with InsuranceWith was £195. That’s Europe and Maldives. The discount code is here: Travel Insurance - Make 2nds Count
Other companies you can try are Insure and Go, Paying too Much, World First or All Clear.