I'm not at all familiar with how Android TV works in terms of updates compared to mobile Android. As far as I know we can't install custom ROMs like we can on phones when they stop getting updates, so I'm not sure what box to get that will get Android TV 12 quickly.
I have a Sony x900f and the only things I want are a slightly faster navigation and Android 12's refresh rate switching/4k UI, but without losing any functionality like Dolby Vision etc. Aside from that, I would go as cheap as possible since I really don't need much from the box as I use the TV as a monitor often, so this is only for occasional movie/show viewing, not constant use.
Android TV 12 is not released yet.
Also, you're asking for a lot of stuff but don't want to pay.
Android 12 won't make your device magically go faster.
Chromecast with google tv if you want cheap. Shield pro 2019 if you want lossless audio