I consider the sad truth is that if you’re stuck on a device that no longer receives updates / is no longer supported by the vendor, you’ll need to be careful with automatic updates. That being said… while you can easily do that on many platforms, from what I understand you cannot control individual app-updates on Roku. Roku will always install the latest available Plex player (unless you disconnect it from the internet – which will impact not only Plex but all your apps).
To be fair to Plex… they cannot keep supporting out-dated platforms endlessly. Maintaining compatibility in new players to old and older server versions might be technically feasible but comes with an increasingly unmanageable amount of work/cost.
I usually recommend users who got stuck on an old platform to keep an eye on the release announcements for their players. Plex usually includes notes if a future version will have different minimum server requirements.
Assuming the NAS itself is still good (which I suppose it is)… your best shot will be to move your PMS to a separate device, using the NAS solely for storage purposes. This could be a spare device or a decently new NUC / Raspberry PI (with some tinkering) / other mini-PC.