Just one game every day, for everyone (like Wordle).
Much less likely to give you a game that has to be solved with guessing.
Arrow keys move around the board. (Or use the hjkl keys if you're into that.)
Space changes the cell from unmarked, to delete, to keep, and back again.
Or, use x and o to mark cells directly
as delete or keep (or unmark them again).
z for undo, y for redo
Autofill: If the sum in a row has been met,
press r to mark all unmarked cells in the row as
"keep". Use c to do the same for columns.
(You can also use f to fill all rows and columns,
but only when negative numbers aren't enabled, since autofill
can't work properly if there are negatives.)
Autofill for X, not just O
Autosave (but then also a restart button)
Save most recent settings as defaults for future games
Disclose errors automatically if someone is way off course (but not right away)