(Threads in the Devices category are generally meant for one device class at a time, so I’ve moved this discussion to Projects so you can get all your questions answered together. 
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Now to specifics…
Aeon micros are good devices, very popular in the community.
Much detailed. Discussion of various sensors in the following thread (this is a clickable link)
Homeseer is a longstanding and well known retail site with a good reputation. There are quite a few others as well–the Deals thread recently had a list. They didn’t cover everything (for example www.zwaveproducts.com is one of the biggest zwave retailers, and www.vesternet.com has a lot of devices from European manufacturers that are hard to find elsewhere), but it’s a good starting point.
You might check the dates on the Aeon siren posts you saw. It’s quite popular in the community, a lot of people have it.
Finally, I am assuming that similar to a traditional security system, I would be able to set up the siren and alerts on a delay from the time motion (or doors opened) was triggered, so that when coming into the house, I would have time to get out my phone and disarm the system??? Kind of clueless as to how that aspect works. Everyone describes trigger the alerts/sirens. No one describes turning them off
Never assume anything about home automation systems, including SmartThings. 
In this particular case, the built-in security function, smart home monitor, does not have any delay, either when entering or exiting. It is a much requested feature. There are, however, community-written security smart apps that do have a delay feature. But their alerts won’t show up on the dashboard in the official mobile app. So it depends on your specific use case.