Dog suddenly aggressive about harness, likely skin related but I don’t know what to do.

My dog has a chronic skin condition caused by hypothyroidism and allergies. It has gotten very bad recently with spring and summer and he is currently dealing with MRSP (dog mrsa, basically). He’s on all sorts of meds to try to combat it.

Either due to irritation from his harness rubbing or just those happen to be spots where his rash is cropping up he has rashes on his side and withers. Either or.

For the first time since adopting him two years ago I was putting him in his harness and as I was buckling it he whipped around with a sudden bark and snap at my hand. I didn’t pinch the skin or anything I buckled it as normal. And I think it’s because of his sensitive skin. No other explanation I can think of.

Switched to a collar which he can tolerate but he has slipped his martingale collar twice in the last two weeks and I just don’t feel safe waking him in it as he could easily run into the street and get hit by a car.

I’m not sure what to do. If his skin irritation is the cause then trying to recondition him to the harness seems like it’ll work…until it hurts his skin again. And no harness seems to not touch that area of his body.
Because of his skin issues I’ve just not walked him for a bit and I feel bad.
Should I be waiting until his skin heals to attempt anything? I feel like a terrible dog owner for not walking him. (But frankly he kind of hates walks anyways)