Well I'm an app nerd so I have a few besides DJI GO4 (in your case DJI FLY)
All this is based on Android ...
AZ Screen Recorder (for recording everything that's on my phone screen when I'm flying, don't use it everytime but once in a while)
Clinometer &
Geometry (to calculate on which altitude & distance the drone goes out of LOS if I'm surrounded by high objects like trees or buildings)
WiFi Analyzer (to check off the surroundings regarding wifi disturbance)
GPS Test (to check the positions of the satellites)
Exposure Calculator (have it set so I can check the exposure values with my phone, convert it to my Mavic Air & add ND filter strengths in the app for the 180 degree rule for video)
Planit Pro (for sun/moon directions ... but most for checking off how the image section looks like on different altitudes ... it has a Mavic Air profile, really accurate for pre-planning)
UAV Forecast (for the weather, KPI values, locked satellites & wind forecasts in different altitudes)
Battery Charge Limit (Require a rooted phone, but it stops the phone to take charge from the RC, still don't prohibit it totally, have mine set to only accept charge from the RC under 15% & charge to 40%)
Well ... that's all