What issues have you found with FUTO? I'm using it RN and beyond Swype being kinda finicky, it works flawlessly.
But yeah, not FOSS. The FOSS keyboard alternatives are indeed atrocious, especially Florisboard which oddly enough seems to be GrapheneOS's favourite option for a future standard keyboard.
Good alternative? Hum, I'll have to disagree with you there.
Firstly, Heliboard it is closer to Gboard than to FUTO, which is closer to Swiftkey. Its actually the reason why I went with FUTO, I need that split keyboard.
Secondly, prediction on Heliboard is just not there. Like, at all. It was absolutely maddening to use, it would predict completely unrelated words even when using custom libraries.
And it actually got worse over time because it started saving nonsense words like "GRT" or "hafe" directly onto my personal dictionary. At one point all I would get was nonsense word after nonsense word. I had to abandon it, it was borderline unusable after a certain point.
Like the commenter above, I too want to sponsor FOSS projects as much as possible, but the offers simply aren't quite there yet.