That's precisely how poll-based opscans work.
Central count (for postal ballots processing) is necessarily different, because that sanity check cannot be done, so voter intent must be adjudicated when ballots (or individual votes) are unreadable. It's a sausage factory.
I know, we have them in my district, but they don't do all the validation I was talking about. I think all that the current machines do is validate that there were no overvotes, etc. I was proposing a separate machine that would let the voter validate that the ballot would be read as they intended.
Although I would favor a screen with physical buttons next to it (not like the garbage you see on ATMs and gas pumps though)
What don't you like about these buttons? As mentioned elsewhere in the comments, this is a proved design that works well for a great number of people. Plus, the elderly / tech averse are likely to already know how it works.
That seems backwards. Touch screens suck. Why not build a validation machine that voters can feed manually-completed optical scan ballots into, before they go to the tabulator? Clear feedback would help catch incorrectly filled out votes before they're cast, no touch screen required.
The validation machine could have a very clear and user-friendly display, which candidate pictures are large type. That would definitely be easier to verify than a computer-generated optical scan ballot.