Custom roms with relocked bootloader only work on pixels by design. You'll have to live with an unlocked bootloader.
As for easy installs, Murena's e/os exists with support. But I can't vouch for their cloud ecosystem. Other than that, maybe an officially supported lineage device. You will lose safetynet on both unless you want to root.
Interesting discussion, but many of the questions have pretty lame default answers. I have a Sony bravia from 2015 for reference.
The TVs that come with an OS instead of just firmware are smart-TVs in all aspects. Your cable TV or hdmi input is an app just like Netflix is, and is subject to a launcher. You can't make it dumb by disabling stuff.
You can mostly reject targeted ads and disable personalized data collection. But smart TVs are priced with ads included, so completely turning off everything will require unsupported modding.
cameras are only found in telepresence hardware, unless you want to be paranoid. Check the feature list. Microphones can be in the remotes of some TVs, but this will usually be advertised as a smart assistent if present.
I haven't seen any TV actively complain about missing wifi (except for during setup for updates)
unless you are tricking the TV into thinking it's online, any connection attempts/power usage would be a bug. Do note that smart-TV will by default have a standby-draw influenced by WoL or similar.
This is pure tinfoil-territory. No hotspot/carrier carries data without being payed for it. It's also not economical when telemetry can be sent over the customers home-wifi in 99% of cases. There is no gain in hiding sim-cards in every TV. Unless you are a person of interest and are sent a modified TV in that case.
Hope this helps.
For now, NC Deck does what I asked. Even if it's bound to Nextcloud, that's at least a server I trust. I also used Markor as my editor ever since switching to Android, it seems to have a todo-focused Markdown extension with linking other files. Looks powerful, maybe I can build something with it.
They do have SQLite on their Roadmap, so maybe there can be a standalone app at some point.
Afaik google-pay is prone to fail even with faked safetynet. Magisk can also fix safetynet, but I don't want to enable root-access. Kinda dumb that the way to fix overcritical security checks is to break security even more. :)
Thanks for the idea though.