My conclusion as well: you can only get M-TTL with Leica-branded OEM flash gear on a Leica camera. I haven't run across any 3rd-party options. It's possible that Leica-branded Panasonic cameras could use four-thirds flash gear for TTL, but for most Leica cameras (M, X, T or SL), the flash hotshoe is incompatible. [Yeah, Leica and Panasonic share the L-mount for lenses, but not a hotshoe].

And even though Leica's own flashes appear to be made by Nissin (e.g., a Leica SF60 looks a lot like an i60A; ditto a SF40 and a Nissin i40, and the SF C1 transmitter is a dead ringer for Nissin's Air 10s), Nissin themselves do not offer "for-Leica" M-TTL versions of that gear at Nissin prices. This was the same deal Leica had with Metz when Metz was making their flashes.

See also:

• Is flash brand X compatible with camera brand Y?
• This comment on l-camera-forum.com

July 2022 update

There are now two announced TTL options for Leica in terms of flash radio transmitters. Profoto is releasing a Leica version of the Connect Pro and Godox is releasing the XproIIL. Neither one is available yet from retailers, but are both on pre-order at B&H at the time of this writing.

But in the case of Godox (at least; may be the same for Profoto) the lights will also probably require a firmware update to be fully TTL compatible with the Leica transmitter, since this is how all new system support has been added in the past, and those firmware updates haven't yet arrived. And there are no announcements yet for on-camera flash TTL options.