In the case of the Xbox, you can replace the mainboard with like a rpi and still achieve it quite easily (I'd say budget ~ 100€). That's harder for a smartphone (I'd say budget ~ 2000€). Either way, the budget is still lower than the price of security flaws to circumvent secure boot.

> Unless you're willing to go full iphone/xbox with no third-party hardware or OSes

And even then, you still have the entire 1st party OS attack surface to play with. Which is just _huge_ specially considering this evil maid scenario implies you have a large amount of time with full control of the device itself and all its hardware.

These evil maid scenarios are so academical in nature by now, that there is practically no way to defend against them outside academia itself.