I have a DSC PC1864 with an Eyezon EnvisaLink EVL-4EZR it is fully integrated with Smartthings using @TAustin edge driver. All contacts and motion sensors can be utilized for routines. I built a home in 2018 which I had pre-wired for all windows and exterior doors. I considered newer more ‘modern’ alarm systems, but realized that modern also meant subscription/proprietary closed systems, that are designed to exclude self monitoring. I had no prior experience with Smartthings. It was included in the new home build.
I would recommend pre-wiring ALL doors (interior and exterior) and windows, as well as garage doors. This obviously will add some cost but will eliminate many z-wave or zigbee sensors, and probably many battery operated devices. Also make sure ethernet is run to the alarm panel.
You may need to add two motion sensors to some rooms as you need coverage for intrusion detection and automation use which may be in the opposite direction to each other. So put as much thought as possible into the layout.
The DSC PC1864 has 64 zones capability, 8 on board and then 8 per additional expansion zone. The reaction time of the alarm sensors in Smartthings is faster than any z-wave or zigbee sensors even on edge (local).
I have a keypad at every entry, by you can probably get by with just one, as each phone will also become a keypad.