For MIDI, the Mssiah cartridge is great fun and includes a sequencer. Supports dual SIDs for 6 voices. https://www.mssiah.com/
If you're not using an Ultimate 1541, you might consider a JiffyDOS upgrade. JiffyDOS is still maintained and sold, and is available as hardware kernal chip replacements. It works *great* with an SD2IEC, and keeps the cartridge port free for other add-ons. The hardware chip includes a switch you can mount to the case to physically disable JiffyDOS for games that don't support it.
If you do use the Ultimate 1541, it comes with fast load cart images built in, and you can also purchase a licensed JiffyDOS ROM image and load it via the Ultimate. I have both an Ultimate and an SD2IEC and use JiffyDOS in this combination without replacing the kernal ROM chip. (When I'm not using the SD2IEC, I just use one of the built-in utility carts.)
For coding, you might reconsider the Ultimate 1541 for its RAM Expansion Unit (REU) emulation. Turbo Macro Pro REU Edition + a real REU or emulated REU is a powerful combination for on-device assembly programming.