The problem I've run into: the buggy software that's 3-6-9 months+ out of date by the time you get it out of box. Offline sofware update is not always an option. Maybe update once and then disconnect? As someone mentioned, only until they have autoconnect built in through partnership with mobile providers

I remember hearing about some smart TV that remembers your WiFi password even if you tell it to forget the network.

I plug my TV into Ethernet to update it and disconnect it immediately.

Does open smart TV software exist?

Kodi?

Disregarding the fucking awful campaign against them because some 3rd party developers make piracy-focused add-ons, as someone who never owned a smart TV, Kodi seems pretty feature-compatible.

I could be totally off though.

Look into LibreELEC, I use that and buy android tv boxes. You can dual boot, LibreELEC from an SD and android from NAND. It's stable enough that I've began to just flash nand and run soley LibreELEC, but you'll be limited from the wide availability of android support for most major content providers apps

the ones from the vendors are all closed source. Best bet is to buy a good TV and use another device to run software that makes it "smart" - ie. AOSP / Kodi

The only thing to gain from "smart tvs" are apps that can be controlled w/ the TV remote, if you're privacy coincious, you should know by now that it will take a bit of effort to secure your data.

In theory Android is open-ish. So lineageos or something might work.