I’m also interested in the Polyend Tracker, but (a) no battery, and (b) it’s still pretty big.
Have you considered the Dirtywave M8 tracker? It definitely fills the role of a portable all-in-one groovebox, just a little bigger than a Pocket Operator, has a battery, seems well-built (you could probably rip the keys off if you tried hard enough, but anyway it comes with a bag AND a case). The only downside is that pre-orders are currently sold out, but I believe there will be more in upcoming months.
You can even try out the headless version now with just a Teensy 4.1 board and a microSD card. I’ve been playing around with it for a few months now and it’s really fun and capable… I pre-ordered as soon as I could.
Elektronauts discussion here: M8 Tracker
Demo clips: x.com