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You could save our time by moving the trackball requirement to the beginning!
First, mint-mobile is specific in some way? That part shouldn't be a problem.
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
On the trackball requirement alone you’re SOL haha. Confusing request, given that pixel 4 doesn’t have one. I think you’d have to go way back to the nexus one for a trackball. I miss those days.
I have been wondering if this is one of those things you could solve with a ~case. Like - clicks keyboard might be overdoing it, but getting a scroll-wheel or a trackball under my thumb on the side, could be nice and doable! :D
I reckon your best bet would be the Unihertz Jelly Star[1] - it has a 3" screen, which is comparable to your old HTC Hero (3.2"), and you can run a LineageOS GSI on it. Oh, and it has a programmable hardware button, 3.5mm jack and a frikking INFRARED PORT to control TVs etc!
Sadly no trackball, but no modern phone has one anymore.
[1] https://www.unihertz.com/products/jelly-star
I've got a Motorola Power G 2024, the cheapest unlocked phone I could get. You can root it, and I use it with Mint Mobile. I've got a VAX/VMS 11/780 that I can run inside it thanks to SimH and Termux. It's not a "power user" phone, per se, but it's amazing what $200 will get you these days.
If those are your "requirements", especially the last two, then no, no phone would work for you. For the first three, you might look into a Fairphone or one that can run GrapheneOS, perhaps the newest Pixel.