I guess the following infos fits here well …
There is one device which is actually pretty genius to make your own sequencer, namely the
Novation Launchpad Pro. They have released a basic firmware for it which provides an empty main event loop and an API to get access to features like sending MIDI, modify the colors of the pads and stuff like this. There is even a small storage space for setting variables (or short sequences I guess).
When you are developer its pretty easy to get your own customized firmware going in a lazy afternoon (depending how much you are already familiar with VM based workflows).
The base firmware for the Novation Launchpad Pro can be found here:
An example of a custom firmware based on it can be found here:
https://fenixsong.com/openlaunch/
Another example - a kind of Controlling Surface for the Digitakt - from fenixsong (but not open source):
https://fenixsong.com/digipad/
Update - see also: