I don't know what Jellyfin or SmartTube is.. but I do watch videos from other sources such as Samba shares, and for that I use Infuse. The Apple TV app is really good and there's a client for macOS and iOS as well.

Infuse app for Jellyfin. It works very well. Afraid I don’t know the other one.

+1 for Infuse, handles streaming Bluray remuxes like a champ and supports Dolby Audio / Vision.

You could try Chromecast. I've used it occasionally, never had any issues with UI, pretty smooth with no lags or crashes. It does have the Jellyfin app on it too.

chromecast (the hardware thingy) has been discontinued

Like me, you use a cheap Kodi box instead of an Apple TV.

But OP's advice still stands: don't connect your TV to the internet.

Swiftfin https://apps.apple.com/us/app/swiftfin/id1604098728

Has it stopped crashing yet? Honest question, I try it every few months and it always spontaneously combusts in between playbacks (browsing media, when finishing playing, etc.)

I do not remember the time it has ever crashed - and I am quite sensitive to software not working well. I can complain about the strange UI choices but not about the stability. Running this on latest Apple TV 4K

A single board PC or a laptop.

There’s a fairly good AppleTV Jellyfin app. It’s free.

Another vote for infuse