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I think a 'Delete' button which would delete the latest in order. So tapping the letters would act like typing on a keyboard, and 'Delete' would like Backspace.

NYT Spelling Bee works like this and it's a pretty similar interface.

You just need a 4th row with a back button to delete, a refresh button to reset, and an OK or check button to submit.

No need for deselecting. Just add Undo and Reset buttons and that'll do it nicely.

For a minimalist version of the same, add just the Undo button and do a full board reset on long-press.

I’d say tapping an unused letter would disable it and grey it out. Then have a backspace button to remove latest letter, re-enabling it in the grid.