"get a Raspberry Pi" is quite difficult nowadays.

RPi with LibreElec is nice.

But inside the TV is the main point. I was wanting something more like OpenWRT or Canon CHDK or similar to control the TV itself.

It's a cool project, but you can always just keep the TV offline and use it for HDMI input only.

Our TCL Roku TV is slow as molasses to turn on and do stuff. Since we solely use it with an Apple TV device, I'd LOVE firmware that simply booted fast and allowed me to switch between inputs.

For that you can get a commercial display with either an Intel board or a Pi CM4... but those displays cost a bit more than normal consumer smart TVs.

Yeah, that's the problem, you're essentially getting less hardware for more money. And if you have to source your own SBC might as well just pick the cheapest TV and connect it via HDMI...