The real power of the ST ADT hub is if you’re going to be using the ADT monitoring.
If you aren’t planning on using then you’re better just using the V3 hub and building you own setup. You don’t have any restriction on what devices you can use. Infact you can even migrate your IRIS devices over to ST including your Keypads, build your own Security setup or use the SHM add on, create your own monitoring rules using CoRE or WebCoRE. The possibilities are endless and you won’t need to use your IRIS setup.