I have procrastinated controlling the opponent's paddle (because it scares me) and reworking my drawing functions for less artifacting. Before, I was clearing and repainting each sprite one after the other. It was hiding the puck when close to my pusher, and artifacting when close to the opponent's. Also it needed a full 8kB background copy.
Each sprite now saves its own background, and I'm doing clears and draws in order so that sprites can overlap. The clear/draw order changes depending on the puck's Y position. It makes the drawing much more satisfying. At the expense of more cycles, but that should fit.
Commit: https://github.com/colinleroy/a2tools/commit/e6e7e8fb40a141dc9e252c42128d43eda7021d07