The biggest question is, are you looking for Dolby Vision support?
There is no open source implementation for Dolby Vision or HDR10+ so if you want to use those formats you are limited to Android/Apple/Amazon streaming boxes.
If you want to avoid the ads from those devices apart from side loading apks to replace home screens or something the only way to get Dolby Vision with Kodi/standard Linux is to buy a CoreELEC supported streaming device and flashing it with CoreELEC.
List of supported devices here
CoreELEC is Kodi based so it limits your player choice, but there are plugins for Plex/Jellyfin if you want to pull from those as back ends.
Personally it is a lot easier to just grab the latest gen Onn 4k Pro from Walmart for $50 and deal with the Google TV ads (never leave my streaming app anyways). Only downside with the Onn is lack of Dolby TrueHD/DTS Master audio output, but it handles AV1, and more Dolby Vision profiles than the Shield does at a much cheaper price. It also handles HDR10+ which the Shield doesn't but that for at isn't nearly as common and many of the big TV brands don't support it anyways.