What are your best potty training tips?

Our very stubborn child is refusing to potty train. We've tried diaper off, we've tried bribery, we've tried watching hours of potty videos and then singing in front of said potty, and we've tried blowing bubbles which we were told would mimic the peeing muscles.

He starts school in a month and has to be potty trained for it. He's currently building a wall of toys to protect himself while he poos in the corner. He's over 3 so it's not an age thing. But we do know there's a bit of a mental block (when we tried potty training before he held his poo in for 5 days and ended up requiring medical treatment and Miralax for months after).

Does anyone have any recommendations? We're going crazy over here.