Mhhhh the Nvidia shield does not really seems like a bad idea… Its price is actually not that bad (even though you need the Pro to have USB ports so i can plug an hard drive into it).
You say that plex on my current FireTv can see and play media from a NAS/other server in the local network?
Compare it to the steam deck (a bit more pricey, but can also run emulators). If you want a more polished nintendo-like experience however, the switch is probably not a bad purchase (consider you have to add to it the hefty cost nintendo puts on their games and the fee for playing online, if you want to).