I do think that congress would impeach if he said "I declare martial law and cancel elections". He'll have to get there piecemeal without creating a single moment for opposition to coalesce.
Why though? Why wouldn't the Republicans just go along with the new dictatorial regime, as they have been every step of the way? If they really had the character you imagine, they would have held the line at the insurrection. Instead they whitewashed it and put him back in power. I am in no way confident that group of people would check his power again.
It'll be much more subtle than that: simply deploy ICE personnel at polling stations who will arrest any non-white person who cannot prove their lawful residence on the spot. They'll be let out, but only after the election is passed.