Your quilt is beautiful. No advise on markers except test it first. It seems that all markers have quirks and don't wash out EVERY time. It seems that fabric has a mind or it's own.
Sue

what ever you choose to mark with you need to test on your project to make sure it will come out- just because it comes out of my quilt does not necessarily mean it will come out- there are no guarantees---always test-
i have had the best luck with a plain ole pencil---i ruined a quilt once with a blue- water erase marker which the lqs swore by-
always test and find what works FOR YOU---and test every time!!! just because it came out this time does not mean it will on a different project- every project is different- and for some reason the same tool does not work on every project

I like the SewLine markers, ceramic lead......... it rubs off easily