Same here actually, ~10 years too lol, since my first Android. SwiftKey worked very well (for me at least)
but its owned by Microsoft, which gathers user data. I've been making a switch to FOSS apps and trying to degoogle myself in the recent months.
It was hard for me to leave SwiftKey due to other options being unsatisfactory / other keyboards always missing something, but HeliBoard did the trick for me :)
HeliBoard is amazing, I recommend it! For those wondering how to enable simultaneous multi-lingual support (i.e. swipe or text in both languages and get predictions in both languages), I explained it in this comment chain, and included a screenshot for reference:
I tried both, AnySoftKeyboard was a bit too much effort and took me longer to type on unfortunately. HeliBoard was a nice replacement for SwiftKey for me though.
Yeah, that's what I interpreted. That's what I enabled with the steps I mentioned. I have the English keyboard enabled in Heliboard. I tapped on it, and added Polish in the "multilingual" section.
I both typed and swiped to właśnie zdanie (this exact sentence) i słowa w nim (and the words in it). Simultaneously.
Added is a screenshot for reference. If you meant something completely different (like letter gesturing?), sorry, I don't follow.
EDIT: And I found it a minute after my post lol. Inside Heliboard, go to settings, find languages, don't use system languages. Add your main language, then tap it and a "hidden" menu will come up. In it is the multilingual option. Tapping it will enable you to add another language to the same Heliboard language.
A cigar and a waffle?