Here is something I did with SHM which might be helpful if you just don’t want the siren for accidental Armed Home Motion events.
I use contacts and motion sensors for armed away. but for armed stay I only use contact sensors. This way the siren only goes off in the middle of the night if a contact sensor is breached. I then have a custom SHM rule that monitors motions sensors and alerts me via text and a bright red hue in my bedroom if there is unexpected motion while in night mode. So, therefore no siren unless a contact is breached.
SHM has a lot of limitations and like you, once I felt that I had enough protection inside my home, I started to look towards securing the outside of my home to hopefully catch things before there was a home intrusion and found that SHM was too restrictive.
Firstly, I made a distinction between outside events that were not intrusions and inside events that indicated an intrusion.
Secondly, I set up SHM to only include those items that indicated an intrusion, ( sensors and switches inside the house).
Thirdly, I use routines to arm/disarm the system. I use Custom Rules to control everything outside the house and notify me if there is any activity there. For example, I use motion detectors to fire my Arlo cameras early enough for them to record all the action, turn on inside lights if necessary, etc.
I hope that helps.
(www.rboyapps.com - Making SmartThings Easy!)
If you aren’t using Scout monitoring or certain feature of SHM like video recording you can use this SmartApp as your Intruder management and it can be defined per mode so you can configure different intruder action settings for different modes. You also have an option for a Silent Alarm if your alarm supports it: