I checked most of the FSL resource to find how to debug a Field Service flow, but couldn't find anything.
Is there simple way to debug FSL flow?
I can understand your pain point.
In my experience with FSL, the best way to debug an FSL flow is to have a screen and print your variables or expected output on the screen.
Login to the FSL app and clear cache to make sure the latest flow with debug screen is available and then launch the flow using actions to see your debug logs.
Hope this helps.