There is a new HDD policy with X25 models and 3rd-party drives. Considering that Syno is advocating for their drives, moving forward, officially, this will be the only disk supported until the compatibility list is expanded with any 3rd party models.
Saying this, a pure migration of drives from x18 to x25 will be possible, but you will have to use the volume "as is". Any changes to it will require a Synology drive from that point forward (or any of many unsupported hacks that will cost you warranty and support. Still, the drives will work, ofc, but a caveat is there).
Now moving past the drive migration, yes, once you get the DSM up and running again, installing Plex on the 425, you are good to go with local access, and with external one, forwarding the Plex port and configuring it for remote access, you will open up communication via the mobile/tablet app.
As
@fredbert already pointed out, there is a "resource" section on the forum that you can search for a number of RP tutorials, including the one where RP is a part of the DSM. Regardless of those few visual changes in the UI, the main thing is that nothing major has happened under the hood, so you will be able to follow along. If you get stuck, write back or PM, and things will get sorted.
Personally, give QC a go and see. Would RP be faster? Yes, but we are not talking about hosting a site that 10k users per hour will visit. RP, while not difficult to maintain, will also have some elements that you are not familiar with that might give you a headache in the long run if it's not automated (SSL cert generation for example). While all things can be automated, it will take a certain learning curve to get there, some trial and error, and again, if this is personal usage within the household, it might be just easier using the QC.
On top of this, with Synology's new BeeStation devices, which work only either in local or via QC (no RP, no DDNS), the uptime and QC worldwide server distribution has increased. There might be some occasional issues with that service, sure, but for the most part it works really well.
The main benefit would be your local Internet connection. The speed of it as well as the technology behind it (DSL, fiber, mobile, Starlink, etc). So, depending on your location, your experience might vary. You mentioned you're from Sin City (I will guess that is Las Vegas), and while I am unfamiliar with network speeds and tech behind it, again, it would be best to just test it out and see if it's too slow or fast enough.
You can always set up one service over RP and see how it compares.