grbzng, thank you!
Using Settick instead of Pause improved the tempo drift somewhat.
However, perhaps because I’m using software UART, the tempo still slows down as the number of notes increases.
I tried to use hardware UART to increase processing speed. But I couldn’t disable the serial console function (which connects to a PC terminal software via USB serial) that occupies it.
This is proving quite difficult…
Reference 1: The “Serial Console” section in the Picomite user manual
Reference 2: Unofficial PicoCalc PicoMite/WebMite firmware release thread (V6.00.02) - #216 by Toml_12953