I've moved to infomaniak too (also using Thunderbird), though I've gone for the first tier of paid account which lets me use my own domain for my email address. Been super happy so far. Instead of firstname.lastname@gmail.com I now have firstname@lastname.uk. Which is better. Might cash up to the next tier which will let me have multiple email addresses so I can let the wife and kids share in the glory.
Only downside is iOS so far. I've got an iPhone 15 which is less than a year old so I won't be changing it anytime soon. Sadly Thunderbird doesn't have an iOS app so I'm using Apple's own mail app for the moment but I'd love to move to a European email app if there are any on iOS. My next phone probably won't be an iPhone. Or an Android.
Any suggestions for an iOS European FOSS email app?