And make sure you understand the False Alarm policies of your monitoring company and local safety services providers. I would recommend that you have some physical way of disabling any sirens you install, just in case. Both SmartThings and ActionTiles require the internet to function properly.
Your current monitored ADT Panel may or may not have had cellular backup functionality.I think this was a feature of Smart Alarm. Hopefully they extend this functionality to future versions or variation on SHM. except on their ADT Security Hub product. You can display your various sensors on your ActionTiles Panel to help but there is no "bypass" option provided by SmartThings. Some let you selectively do a one-time "bypass" of these sensors so you can just get on with your day. One thing that traditional alarm panels usually offer is a pre-arming "zone-alert" that indicates a window or door is not currently closed or there is motion detected somewhere in the house.
You can use a PIN Protected Virtual Switch Tile to control such SmartApps. You can also use an alternative Alarm SmartApp such as Smart Alarm (in the Classic App "Marketplace"), or perhaps one that has been written as a webCoRE piston, etc.While you can use this App for other features, you can have the SmartThings Classic App still installed and usable for configuring and managing Classic SHM. ActionTiles is not currently compatible with SHM in the new SmartThings App.I presume comments on that SmartThings Community Topic will give a good indication as to reliability. ActionTiles's PIN Protected SHM Tile, in concert with the SHM Delay SmartApp from the SmartThings Community, should reliably replicate the entry/exit grace delay behavior.