Showing contempt for him at this stage could be interpreted as contempt for these issues.

Eh, that seems equivalent to saying gardening can backfire because you could stab yourself in the foot with a pitchfork or something. You can show contempt for their solutions while acknowledging the problems. Obviously, if you suck at rhetoric you shouldn’t get into debates in the first place.

Using logical fallacies against him is to basically hand him that legitimacy for free by making him appear “reasonable”.

The sort of people who are swayed by fascist rhetoric aren’t the sort that have a keen eye for reasonableness and logical fallacy. It’s an emotional stance based on posturing. Weakness is a far greater sin to the fascist than irrationality.

The easiest solution to all this is not to debate them at all.

That’s not a solution at all. If no one pushes back on their messaging, they get implicit control of the narrative. That’s a lot harder to do when their audience tunes in to watch their idol own the libs, but then see him get dog walked.