If you want to catch changes to combat mode to apply settings before combat then I would use PLAYER_REGEN_DISABLED and then reverse it out on PLAYER_REGEN_ENABLED.
I use these to stop interactions during combat that players may be tempted to do or to minimise/hide non combat windows during combat. PLAYER_REGEN_DISABLED may trigger after UNIT_ENTERING_VEHICLE or before. Similarly PLAYER_REGEN_ENABLED may trigger before UNIT_EXITING_VEHICLE or after.
Not too much helpful but they may be events you hadn't considered before.
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