I’ve been happily dualbooting Win10/Mint for years on the very machine I’m writing from. Zero issues with boots, GRUB, no need for flatpaks, both systems work fine, both configs heavily customized.

What was your starting point? Having Windows on the machine already and installing Ubuntu on top of it? (that would be commonly regarded as the ‘correct’ order) If not, and you can afford to scrap everything, that’s what I’d go for. If yes, and it’s Ubuntu that’s messing up your dual boot, I’d see if Windows can be restored after removing Ubuntu, and then try a different distro (I’d go with Mint Cinnamon, it normally provides smooth install experience).

What’s your long-term experience?

Excellent. After uninstalling it never comes back.