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Search the Forum for “IRIS Keypad”. It is inexpensive and will interface with the Arm/Disarm functions of SmartThings Smart Home Monitor and more.
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Consider ActionTiles… The web browser based dashboard panel builder for SmartThings. It offers a PIN Protected SHM control Tile and much more.
Note that both solutions above do not solve the problem of not exit & entry grace delays, but there are SmartApps (etc.) you can add when you get to that point. Search this Forum, please!