I have both Hubitat Safety Monitor (HSM) setup and Ring Alarm. You are correct that you can add and setup motion and door sensors and then get alarms via a siren, and also via notifications on your phone devices, Alexa Echo, Google home, etc.
There is a community integration to Ring that works great but you need to remember that it could stop working if Amazon/Ring change something that impacts it. I currently use it and love it.
Quite a few people do that. You can use Sharptools.io. Smartly is an external integration where you add your dashboard and it allows you to add other features to it.
There are a lot of options! I personally really like the Inovelli switches. My switches are either Inovelli or Lutron. For Lutron, a Lutron Pro hub is required, but well worth it!
I love the Samsung Smart Things Motion and Contact sensors, but they are not available right now... I understand that Aeotec will be producing them soon. I also really like the Inovelli motion sensors.
You will get a lot of options from the community. Welcome!