Ah, darn. Unfortunately I have no additional help to offer since that particular issue was fixed for me after changing those options in Flatseal.
I’d try running Firefox from the terminal to see what error message you’re receiving when the crashes occur; the unique error message was what led me to this workaround when I was originally troubleshooting.
In my experience, you don’t even need the dedicated Jellyfin client. Just opening it up in a web browser works out of the box, so that’s potentially one less thing to download/install/manage for the clients.
That said, I’ve never tried to access Jellyfin from an iPad/iPhone/Mac so it might not be as seamless as my experiences on Android/Linux based devices. But I imagine they’d be fine; just test it out before you hit the road.