I've used MessageEase for years now on Android and it was really worth the time investment. I only switch back to Gboard occasionally to get at its far superior emoji input system, but I don't use a lot of emoji in most of my text entry so that's not a massive deal. I can type accurately without having to wrestle with autocorrect all the time, and that'll do for me! And although the letter frequency distribution isn't right, I can happily use it to enter German and Lojban and Welsh on the occasions when I want to use those on my phone.

I haven't gone too far myself, but I'm pretty sure it's possible to customize the symbol layout to better suit a particular language or user preference.

IIRC you can swap out (or add) any of the side/minor triggers, but I don't recall if it's possible to change the big nine major keys.

It is great though, I abhor word prediction when I'm trying to write (the smarter the predictions, the more viscerally disturbing I find it), and ME has been the only way I can keep up a comfortable pace.