Have zero knowledge of Roku but for example with AppleTV boxes it is totally fire and forget, Tailscale is always on in the background and apps will find my media servers through it.
And I’ve noticed even tech illiterate people will learn to become literate when there is some motivation, like a huge movie archive :) Flipping a preference like VPN on if they want to use certain app is certainly within realm of possibility.
Why exactly would they ever need to turn VPN off again? It’s not like all their traffic will go through it if it’s on, unless you specifically configure stuff that way (exit node, routes).
And one option to do the VPN stuff is on their router too, so it’s totally transparent to them. More stuff to configure though, unless running owrt or some other router software compatible with Tailscale.