Empty parking lot practice is very good. I've done exercises with cones with my stepson. He has some orange cones for sports, and I've set them up in a totally empty parking lot (weekday business with zero cars on the weekend) to guide him in exercises involving a straight line, braking quickly, and weaving around. Just don't overdo it, though. Stepson was doing really well, but was getting tired and wanted to show off his skills to dad at the end. He ended up wiping out and being scared of hard surfaces.

Even after 20+ years of bicycle commuting in various ways, I still will sometimes go to an empty parking lot to practice. I practice leaning, weaving, counterbalancing, slipping between curbs, and stopping at an angle/turn (the key is to straighten up fast and then hit the brakes). I also like to see how slow I can go while maintaining balance. There was a point several years ago when I could remain upright with feet on the pedals at a dead stop. (Not that good anymore, though.) If you're feeling really confident, you might try going without hands (while moving and while in the empty lot). It helps you to feel the bike beneath you and respond more with your whole body, instead of trying to steer it like a car. (Just put those hands right back on the bars and go straight when you feel unsteady, though.)

The only other idea that I have is to get a heavier bike. For me, balance has always been easier on heavier bikes. I think it may be due to the fact that you can be a little off and still balance when there is more weight involved. It also worked for stepson, who couldn't balance a bicycle at all until he got a big, heavy one.

I respect what you are doing, because I know that two bicycle wheels are simply much more difficult for some. The fact that you are willing to work on perfecting the skill rather than giving up is highly admirable. You can get there!