Since I'm also a Canon shooter, I just use a Canon 600EX-RT set to manual mode if I have a bit of time to determine the exposure and/or if the situation remains relatively consistent (same ceiling height throughout for bounce flash, for example). Alternatively, I'll use the thyristor option to dial in a specific aperture value and it will then automatically adjust output from the flash based on subject distance. Unfortunately the TTL flash options for Leica are rather limited. A flash that looks appealing is the Nissin i40, though it won't support TTL on an M, so looks like you'll be stuck with manual power adjustments. I don't think it has the auto thyristor mode, which was something very common on most flashes until TTL became mainstream. Now it's tough to find that specific feature. I guess a Vivitar 285 would be an option, though back to the same size as the big flashes...