We are similar in pursuits and age. We are different in that I succumbed to the "Leica M Allure" at the start of my photo trek, rather than later. I have used one ever since.
Like you, I recently backed off doing so many weddings ... but still do them occasionally. Also do a fair amount of portrait work and some model portfolios, etc., and of course personal photography.
I recently sold off my entire Sony A99 system except the ZA24-70/2.8 which I now use on a A7R with LAEA-4 adapter for AF work.
I also use a Leica M Monochrome Rangefinder, and a Leica S(006) with 6 lenses.
For your consideration:
None of the M or S Leica's are all that great at higher ISOs in difficult light, at least not noiseless at 1600 in the way you want them to be. Your Sony A7-II is most likely better at higher ISOs, and most certainly faster at focusing unless you are extremely practiced at Rangefinder manual focusing.
The reason to own a Leica M is for the rangefinder manner of making photos, which is a different way of seeing the world around you. Different, not necessarily better, depending on your end goals. The other reason is the Leica M lenses. The Leica Rangefinder is "legendary" because so many famous photographs were taken with one ... however, study the type of images those famous shots are ... it'll provide a hint as to more suitable rangefinder use ... and how you may employ it for certain types of work you do.
The reason to have a Leica S is how diverse it can be. Unlike the Hasselblad V and H or Mamiya, Contax, and Pentax cameras, it has a dual shutter system that allows use of the CS lenses with leaf shutters that sync up to 1/1000 with strobes, or use of a focal plane shutter up to 1/4000 with the same lenses. This makes the S a wonderful kit for studio and location work ... with lighting, or without.
Recommendations:
The S lenses are second to none, but are quite expensive, so I would NOT recommend stepping into that financial quagmire just because there are some S cameras at a "reasonable" price. Commercial work paid for my S kit, otherwise I wouldn't pay that kind of money for occasional wedding or portrait work.
Besides, Leica is not an iconic brand because of the S system ... the Rangefinder is the source of the "Legend" and would provide a new photographic approach for you to explore.
Look into getting a Leica M9. The M9 has a cult following because it has a CCD based FF sensor that many feel has a special look and feel that has more "Leica Like POP!" than newer M digital models. This notion irritates owners of M240s, but many die-in-the-wool M users subscribe to it none-the-less. Irregardless, a M9 will offer a different experience as well as "look and feel" from the Sony cameras you use ... and more importantly, it better fits your budget.
Keep your Sony A7-II for AF work where needed, and higher ISO stuff (certain shots at weddings and difficult light), and get a M-to-E adapter to use select M lenses on the A7-II. So you would have 2 small cameras to use M lenses on. The combined 2 camera kit with a AF FE lens, and a few smaller M lenses would be less bulky and lighter than the A99 and zoom.
Members here can tell you which M lenses also work well on the Sony A7 series cameras (not all do) ... to start I'd ask them if a M 50/2 Summicron performs well on the A7-II. I use a M35/1.4 ASPH and a compact and excellent, yet inexpensive, M90/2.8 for portraits on my Monochrome and my A7R.
Good luck on your new adventure!
- Marc
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