Sorry to pop your bubble, but the moment you add any IP connectivity it will be eventually used to mandate internet access. The devices you have now might not yet demand it, but you will have to replace them at some point or extend your setup in the future. We've seen this play out so many times now, it's baffling people still think this time will be different.
There's a saying: If you set up a trough, the pigs will come.
There's a saying: If you set up a trough, the pigs will come.