I made the same transition as you, and for the same reason - increasing difficulty in focusing using Leica M cameras. In my case I opted for the original SL) if for no other reason than that one turned up at a local camera store and the change over from my Leica M was very reasonable indeed. I can make a few comments about my experiences with it.
- I think even my original model SL is a very useful step up from the compromises one must make when using an M - which is 70 year old technology (even a digital one.) It is faster, more flexible and while there is a learning curve naturally this in fact is very short - the basics can be gained in a couple of days easily.
- I love to shoot old lenses. Including Leica M glass, Leica LTM, Canon LTM, Nikon Rangefinder glass and a wide variety of SLR lens marques. All of these can be successfully used with just the addition of a relatively cheap adapter. (Adapters generally run from very cheap to quite expensive but I find that even the moderately priced ones are very good - certainly good enough.) In fact, for convenience I have recently swapped to a system of habitually using a Leica M to Leica L mount adapter on my SL body and mounting a corresponding Leica M to whatever mount type ta specific lens requires.) Thereby making all lens types a form of Leica M mount lens. This is remarkably successful even though it involves stacking adapters unless of course I am mounting a native M mount lens. (Very, very occasionally I have had an issue with tolerances of the adapters being out enough to create issues with infinity focus but this is very rare.) Some people advocate only buying the most expensive lens adapters available (some of these can run to several hundred dollars (as of course does the Leica made M to L adapter) but I find it is not really necessary. Though if it "floats your boat" go for it. One Leica L to Leica M mount adapter I did buy recently is a Light, Lens Lab close focusing adapter (i.e. with a helicoid) for Leica M lenses. This provides a full range of correct focus from close up to infinity and the adapter is extremely well engineered. Not all such close focus adapters will provide a full range of focus however so be aware. It cost me about $100 USD.
- Having said all of this you might correctly surmise that the SL works well with Leica M glass.
- My SL's focus peaking is quite good though not world leading (it is after all now a 8/10 year-old camera body). But with image zooming as a further option it all works very well. You certainly get neither with a Leica M.
- The SL lacks in body stabilization. I regard this as perhaps its biggest shortcoming as I have been spoiled by other camera types that have it too. I believe later versions of the SL have it, including the SL2. For me this would be perhaps the only reason to upgrade to an SL2 as there is no doubt it is an asset in some situations.
- My SL has a dynamic range of 13.4 Ev. Later bodies are even more (up to 15 Ev I believe). Its ISO settings run from 50 ISO to 50,000 ISO. I have not done much night shooting or low light shooting but only a few days ago took it with me on a night walk. Its low light performance I thought to be very good, as might be expected. My initial feeling was that its low light performance was, however, a little less than the other low light "Queen" I own - also an older camera of similar vintage, the Sony A7s (original version) the ISO of which is said to exceed an insane 400,000. I have shot it at I think 100,000 just for a test and was impressed with the outcome - a photo of the back yard in nothing but moonlight and it did well. But then again, this is probably an unfair comparison - the image size on the A7S is a mere 12 megapixel compared with double that for my SL. I cannot say if the SL2/3 would do even better.
- I use auto ISO and as a matter of practice have kept the max to 12,600 ISO on my SL camera (I find that any more is superfluous in anything but the most extreme circumstances) and upon enlargement of the images there was some noticeable noise (both color and mono) but it was not egregious and also cleaned up very nicely in post processing - modern image editors have become very good at this.) So over all I can predict you should be happy with any of these camera's low light performance even if shooting at night. I will post two shots below taken with this camera on that night just to show you what I got with this and a Leica M lens Bear in mind however that I used a Leica Summarit from the mid 1950s and though I shot at f2 not f1.5 the images are characteristically low in contrast and color saturation, and have some blooming from bright lights - all of these are lens issues (which actually I rather like) not camera issues. I did not check at the time but anticipate that they would have been made at 12,600 ISO as this is the max auto ISO I had set at the time.
- A bit of an afterthought. If you want good outcomes, I always advocate shooting RAW files and post processing for best outcomes with any modern digital camera. Especially if you like low light images - JPGs out of camera just do not perform well in that environment on any system. The SL series is no exception. If shooting film presently this may mean learning some new skills.
Despite comparatively bright light sources (which look much brighter and more effective than they were in reality) the restaurant was really very dim - I should know I eat there quite often at night with friends.
