While I can't offer any advice regarding the paint, I can give you a different option for your boots altogether!
When my friend and I have to make boots (and she ultimately made the Saturn boots for our senshi group) we usually buy a pair of plain boots we can work with and glue on garment leather or high quality vinyl with E6000. It takes several days because you have to hold each piece until the glue sets, but it's totally worth the effort! Use some scrap fabric to experiment size and cuts to make your pattern before cutting the real fabric. Then you can grommet and lace the vinyl or leather after! For Saturn boots, find a nice plain pair of cheap knee high boots to work with, preferably with a zipper on the inside. The only boots we could find at the time were stretch on so it added a few complications.
Once you've patterned out your pieces you can add stitch details and stuff before gluing them on if you really want to add another level of details. You can double up on fabric or use leather cords for details. There's a lot of good customizable options when you go this route. And then you don't have to worry about flaking paint. At most, every couple of wears you might have to glue a loose piece. We wore our original senshi costume pretty often for about 2 years and I think only one small part on one boot could use a little more glue.
Fear the storm, it's coming.