Macropads are a whole big thing in the mechanical keyboard community. You can get one that runs QMK, you can put encoders on them, have many layers, etc.

https://mechwild.com/product/murphpad/

https://knob-goblin.com/

https://github.com/dekuNukem/duckyPad

https://www.gboards.ca/product/faunchpad

It's also worth looking into small ortholinear boards where you can do the same things, but the form factor is more horizontal or has more keys.

https://mechwild.com/product/big-dill-extended-bde/

https://www.gboards.ca/product/butter-stick-limited-edition

https://github.com/nicinabox/lets-split-guide