here are some requirements that i can think of:
- not huge. small enough to use with 1 hand
- able to root and install custom OS without too much difficulty/annoyance (the manufacturer doesn't actively try to stop you from modifying your own device)
- able to use mint mobile
- has a trackball (like the htc hero, that thing was fuckin awesome)
- possibly some hardware buttons
Second, you mostly get to choose one.
Small enough? Yeah, Unihertz Jelly Star is tiny. Maybe you try one of the foldable flip-phones, Razr 2025 allows you to mostly live in the outside screen.
Custom os? Is there anybody else than Fairphone these days?
Buttons? Unihertz Titan emulates old blackberry passports, so it might be too big for you.
I recently bought Galaxy Fold 6 and live inside of the Nixdroid terminal :D
Sadly no trackball, but no modern phone has one anymore.
[1] https://www.unihertz.com/products/jelly-star