One thing I was thinking about is to have 2 parts on the SD card. The whole sequence in a stream way, so its like playing a .midi file. And another structure which is the sequence in parts: song, patterns, events.
So, when you record or edit something, the code must update the complete-sequence. That could slow things down. But if I recall right, my old Roland MC-50 worked like that. Every time I changed something, it would do some sort of process. Just simple things, like event changes, wouldn't do that. But moving things around would...
Wk