I FINALLY got bankswitching to work on GoSub. It is now a 64k game. I'm up to 13 levels designed and put in the game. I am aiming for 30, an ending, and an Easter egg. I am only using about 40% of my total space, but now that I understand bankswitching, I can always just add more space if I need to. Provided the levels I am making don't get too complicated because I only have about 4% of the bank where I am putting in the main game left.
Most of the recent time has been figuring out how to make the sub go off the screen and reappear at the other side. It took a lot of number guessing and correcting, but I think I have it good now. This will help me get to 30 levels better since all the levels I was designing up to this point seemed eerily similar.
I also got the new crash to sound the same no matter what music was playing. It turned out I wasn't writing to NR21_REG, which partly describes channel 2's "instrument," so to speak. It was just borrowing whatever value was in the music tracker at the time.