https://f-droid.org/en/packages/org.dslul.openboard.inputmethod.latin/

Expect a wave of fingers cut off and eyes picked together with a stolen phone? Or kidnappings more likely?

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).

For starters you might want to check privatezilla and bloatbox by Bel: https://www.builtbybel.com/apps https://github.com/builtbybel

Check out /e/ and their Murena OS: https://doc.e.foundation/devices

Mozilla deals with Google

With how much revenue comes from those deals, we might say it's practically financed by Google. FF is more Google than Chromium-based Brave if you follow the money.

Yes, you can.

Waterproofing requirement means that glue is mandatory even with the presence of a gasket

Huh?