Is just adding an SEM (which currently looks sold out) to this system the right way to go here? If we have the mobile app, I don’t know if we would benefit too much from a more sophisticated panel, but maybe there’s something I’m missing?
There are a couple options here. The first would be to just add the SEM, which would allow you to use the system as it is.
Here is a compatibility and installation guide for the SEM:
Another option would be to replace the panel that supports 433Mhz sensors, like the 433 version of the IQ Panel 4. If you want to upgrade, look for the IQP4002.
Adding devices and ongoing maintenance is easier with a Qolsys panel generally, but if you do not plan on adding more devices, and the system in place is functioning well, the best option may be the SEM, as it would give you remote access with minimal changes to the system.