There are a few midi sequencers for linux/raspberry… For the “zynthian purpose”, I would like a friendly step sequencer in combination with a good controller surface. But, with a low cost Arturia beatstep, we get the same features without coding anything.
With the features you mentioned @FransK , it seems to me that you are looking for something like an arranger software, just like the Yamaha PSR line.
Look this: