Nostalgia

edit: oops I just realized that you seem to be referring to "how can I see the letter on the keyboard", originally I thought you meant "how can the keyboard know which letter I'm referring to if it's ambiguous". For me being able to see, I roughly know where all the letters are out of habit, so it's more muscle memory than having to look. But also my finger doesn't tend to cover most of the screen, so maybe I can't see letters one key away from my thumb, but I can see all the others.

TL;DR: try to do curves instead of straight lines when swiping, it helps signal that you are avoiding the letters in a straight line between two letters. I think this is essential but I'm not sure if it's really communicated anymore.

ah, voice to text also seems really convenient. I tend to prefer being silent, but if I did more hands-free stuff then maybe I'd get more comfortable using voice to text.

With swiping, you're right that it can be a bit ambiguous if you just move your finger in a straight line from each letter you need. There are some words that often get mixed up if you do that. What I do is make a curve between letters, especially when they are close together or seem to get mixed up (or there's a letter in between that could plausibly be added between your two endpoints). So instead of going straight from "F" to "L" on a QWERTY keyboard, I'll do a half circle almost, curving down to "N" and back up to "L". This might be a bad example because it doesn't look like there are a ton of common letter combinations between "F" and "L", I can't think of any right now. But when it's especially ambiguous or close together, I think the curving helps. Also over exaggerating sometimes helps, sometimes if I was swiping to the letter "A" it would use "S" or something a bit closer. I think I was not swiping far enough, and I think there is a lot of prediction at play to figure out what is most likely based on your gestures. Overall this works pretty well, now that I'm used to it, I can't recall any specific words that are always messed up. Mistakes do happen sometimes but generally I feel like it's faster than any alternatives (with the possible exception of voice, but I find that I rely a lot on punctuation that isn't always captured by voice).

RE the trail, I guess I don't notice it anymore. It actually works well enough that I don't need to look at the keyboard very much when doing it, I tend to just look at the output, and only look at the keyboard if it entered the wrong word. (And I do occasionally just press individual keys if I'm entering a word that isn't in the swipe dictionary). I'd also guess that you might be able to disable the trail, there are usually configurable keyboard settings.

But then again, it might not be everyone's thing. I had tried T9 on flip phones and never liked it, but I was used to my small QWERTY flip phone. I certainly hope that they let people swiping it if it gets in the way, I don't think it should be forced on everyone just because I strongly prefer it.