i got a mint OP12R, granted i probably shouleve gotten a new one, but it dint want to spend too much, 100 more for a new one. i would say go to places like swappa and check out different phones, and you can ask specific question pertaining to the phones.

While, I'm not sure if you're in the US or elsewhere, here is my response from a US perspective. Another consideration is what carrier do you use, especially if you're in the US. A phone with a user-replaceable battery that is under $300 is going to be hard to find, at least new.

My recommendation for a new phone would be the CMF Phone 1. While the battery isn't user-replaceable in the "pop out" sense, it is designed with a more accessible battery for easier replacement than most modern smartphones. https://us.nothing.tech/products/cmf-phone-1

For used, this is also challenging. If you can find a Fairphone 3 or 4 in the US that works with your carrier, that would be perfect. The Fairphones have batteries that are removable by popping off the back cover. They are typically around $300 I think.

No replaceable battery but any cheap Pixel with GrapheneOS. It's secure, no Google by default, has the most standard UI possible, and Android Auto works great. This combo is the iPhone of Android IMHO. Some applications may not work for some reason but I haven't met those yet.

It may explode soon? I have a 6a, as well. Is that something I should be worried about?

Fairphone?

If the battery is expanding you should contact support. If you bought it from Google they might replace it. They did for mine out of warranty after seeing some pictures.

I would get a new battery and throw GrapheneOS on that bad boy.

There's always the Moto G Power. It's $300 but the battery isn't the most accessible

The pixels are pretty good. U can put GrapheneOS on em and they are reasonably repairable as well.

Used S23 is in your budget

Gonna be honest, kinda bummed. I cant seem to get a good phone that lasts. Every 2 years Im getting a new phone...I kinda hate it. I thought the 6a with the extended software support was going to last at least 4 but it seems its not even going to last 2 years.

9a

you should aim at $100ish (used obv) and make it as fungible as possible. a lost or stolen or exploded phone is then a no biggie. it's encrypted by default so nobody can get at your shit, it's easily replaced with another one, restore data, roll on. so it doesn't matter which one you get, as long it has official lineageOS support. go to https://wiki.lineageos.org/devices click on "filter" and choose the latest LOS version (22.2 as of now) and then pick and choose whatever's in that budget.

edit: replacing the battery on the 6a is also possible, youtube for details; not the easiest thing out there but not that hard either, especially when it's doomed anyhow.

Exploding batteries is just Li-ion doing its thing and succumbing to thermal runaway while at it. Bad battery batches are common. Just replace it.