Training harness avoidance while living in apartment

Like the title says, I’m currently living in an apartment with a 2.5yr old Pomeranian. Every time I pick up the harness and leash he runs away.

Before I brought him to the apartment, I was at my moms house where he had a yard and could do his business there so we worked on desensitizing the harness and leash and he’d let me put it on and off with no issues in one specific room only (where I would do mini sessions of treating him heavily to change the condition of the harness). I thought it worked but never actually took him out with the harness and leash since I didn’t have to force it.

But now in the apartment we HAVE to go out to potty and he is back to running away from it. I’m not sure what to do as now there’s no where for him to potty if it’s not outside on leash. He doesn’t mind the harness once it’s on and the leash he’d still prefer to not have it but he’s generally okay once they’re on and we’re outside. It’s the getting it on part that’s a struggle.

I don’t know what to do. I’ve tried doing mini treat sessions with the leash and harness when we don’t have to go outside but then there’s times when I absolutely have to take him or he won’t get a potty chance and it’s like all the work is undone. In the apartment he has slowly gotten close to the harness with treating but doesn’t let me put it over head. I have to corner him to put it on when we HAVE to go out. Any advice is appreciated!