Wow, as others have said below, disabling the “Slide to Type” feature in Settings > General > Keyboard makes typing work well again on iOS. I cannot believe I put up with this awful typing experience for the past year/years. This should be broadcast more widely somehow. I’m sure many people have just assumed they got worse at typing. I am genuinely flabbergasted.

HOLY shit. You just changed my life. You're absolutely right. It fixed the exact problem I could never quite pin down. I guess the keyboard was always a bit too eager to detect a swipe? This is absolutely nuts

edit: i can type without looking again! i hate that this was the issue.