I want to unlock the ability to view content on my mobile device. I can do that with a one-time purchase (payed with google opinion rewards) or with a subscription. I want to unlock the ability for my entire family though, across multiple devices. Do I have to pay the “one-time payment” for every device? If so, the plex subscription might be worth it. Is there any other major benefit to having a plex subscription?
If you pay the one time fee or the subscription fee, the system unlocks all the paid features for all of the users on your account. Under your account you can have multiple users, each with their own viewing history (and restrictions, iirc). So if your users log into Plex on their devices using your account username and password - abbadon420 and Hunter1 - and then select their user profile, they will all have access to the features.
The one time fee unlocks forever (the lifetime of Plex, of course) and the subscription unlocks the features for the term of the subscription and then you revert back.
Also make sure to set up 2fa on your login