Supported devices are usually older models, and often there are newer devices
Usually. If you want a new phone, your best bet is Google’s Pixels, OnePlus (although recently, it’s been weird), Nothing probably too. Google’s phones in particular have a long history of being really easy to develop for, you can pretty much just build AOSP and it just works. They’re community supported for a really long time.
Most other brands make unlocking their bootloaders either hard or impossible, and to even just root, you need to unlock the bootloader anyway, at least the way without exploits. Since unlocking the bootloader is the big hurdle, root and custom roms tend to come together. So a phone you can root to debloat as your second option would probably also be suitable for option 1.
Sure, it’s in the pricier range, but if you value the ability to control your operating system as much as I do, it’s worth the price. Plus, I think my OnePlus 8T will hold up great for the foreseeable future. It’s way more powerful than I need, so I’m sure it’ll take a few more versions of Android. I’m using 4-5GB out of the 12 it’s got on average, they’d have to seriously bloat up AOSP to use all that. I’ve dealt with cheaper devices and they just kinda suck, stock or not.