I can totally sympathise OP, I’ve just come out the other side of something similar with my youngest who’s 4 later this month. Total refusal to wear pants, sit on a potty, look at a potty, discuss a potty! Our issue was time, we tried to start many times, he refused, kicked up a fuss for a few days and we ran out of time, had to go back to work/ nursery so that was that.
I’ve just had a week off work and, like you, reached the end of my tether so went cold turkey on nappies and resolved to not go backwards. The first 3 days were hell. Tantrums, screaming, kicking, withholding to the point he was clearly in pain, he thought if he created enough we’d back down as we had before. Then on day 4 he started to come around, and from there it was a dream. A week in we’re now on our 4th day with no accidents, and he’s absolutely fine about it all.
Obviously if there is some deeper rooted sensory issue there this is absolutely no use to you, but if not, it may be worth just gritting your teeth, accepting it will be awful for a while and doing it anyway. I’m sorry your GP was so crap, if you think there’s something else that you need it may be worth talking to ERIC, I never got as far as calling but their website has a lot of great information on there and I’ve heard good things about them.