To take this a step further. I want a phone that is small (doesn't have to be tiny, just iPhone SE 2020 or smaller, please), has a replaceable battery, has an unlocked bootloader, has a headphone jack, and costs $400 or less.

It doesn't need to have a cutting-edge processor or tons of RAM and storage space or a 120hz screen or razor-thin bezels or a studio-worthy camera, yet somehow all these things are prioritized on the market over a basic, reliable phone.

I guarantee you that, given your requirements, this will never be a product that you can buy.

Hardware projects live and die on scale. The engineering and tooling costs are a similar order of magnitude whether you make 1000 phones or 1,000,000. If you can guarantee that you have an accessible market for a million devices, then you're starting to get into the region of scale where this would be an OK idea.

Mind you, that's a million users who are cool with all the design tradeoffs you had to make to ingress protection, software performance with modern android, and form factor in order to get your desirable characteristic.

The Punkt MP02 is at roughly the price point and "niche-ness" as the product you describe here, and that sold for almost $400. They could afford to build in about the same amount of functionality as a Nokia brick of yore (but with 4G radios!) for that price.

Whenever someone tries to build a phone that even tries to tick those boxes y'all just find new excuses to not actually pay for it.

You should check the phones from Unihertz, “the worlds smallest smartphone”

there are websites made for you with millions of parameters to find the phone you need. not amazon or ebay

The motorola razr flip phones are great in my opinion.

Have you looked at Motorola? I'm not sure they have all of those features, but me and you think similarly and when I did research, I ended up choosing their $130 phone for my contractors.

But I main the $900 pixel.

They are so similar its weird, but Motorola was slow with snapchat and the keyboard some time.

Most 2012 era used phones will work here. Pick one off eBay.