At the risk of once again being a dummy, I would ask "Does not the Leica M7 have a pc connector"? If it does have a pc connector then off camera flash with an Leica M7 is usuable with the great majority of electronic flashes thru the use of a long pc cord.

To me the most flexible use of flash comes from the units that offer a power ratio control from full power to 1/32 power or less in a stepless fashion. These units used skillfuly are fully capable of doing most of the things that a highly automated electronic flash can. It does though pass the requirement from a circuit board and sensor to a photographer's skill and experience.