I have one of those cheap TCL Roku TVs, I think it was something like $130 for a 55 inch a few years ago on sale.
I put it on a VLAN then added a rule just for it to only ever allow communication to my Jellyfin server. No internet, no other devices, Wireless AP isolation, just able to access that port on that address.
It works ok but it does nag me that it’s not connected when opening Jellyfin, cheaper than adding a dedicated TV box to it though! I tried a similar setup with an old Fire TV and it really doesn’t like being “offline” like that and makes you navigate through the settings to open any of the apps.
If I were buying new I’d try to go for a monitor for sure, it’s just not worth all the potential issues.