Raspberry Pi is actively preventing enthusiasts from upgrading soldered memory to a larger configuration even when the replacement chip is technically compatible. The company says the restriction is intended to discourage unofficial high-capacity resales.
For a long time I was using a tegra dev kit as an HTPC. Definitely not the cheapest option but I got it for free so it worked out great. I bet you can get the old TX1 boards pretty cheap these days, and they just run Ubuntu with some Tegra mods, but apparently can also run NIXOS as well.