Follow-up comment, since one of the comments mentioned Nvidia Shield: I just realized LineageOS can be installed on various Nvidia Jetson devices: https://wiki.lineageos.org/devices/#nvidia
This seems to match my requirement of "hackable" quite well, though Nvidia Jetsons are obviously not "fully open" or anything and a quick search yields various threads of people struggling a bit to get LineageOS runnning. But it does seem possible! (Not sure whether Netflix or Prime would run, though.)
(In fact, there are also inofficial LOS builds for Raspberry Pi, too: https://konstakang.com/devices/rpi4/ )
This seems to match my requirement of "hackable" quite well, though Nvidia Jetsons are obviously not "fully open" or anything and a quick search yields various threads of people struggling a bit to get LineageOS runnning. But it does seem possible! (Not sure whether Netflix or Prime would run, though.)