I've used a 24 mm Nikkor for decades for everything from weddings to news reportage. I wouldn't be without one.
For the last 8 years I have been using a Leica 21mm f2.8 ASPH on my M7's.
True the RF is easier to focus but with 24-21mm, unless close in you can zone focus easily.
Hands down the SLR is much easier to workwith for framing and alignment to avoid the dreaded curved linear lines and skewed focus. Indeed an external finder on an M MUST be offset because the hot shoe on M cameras IS NOT overthe lens centerline and you will geta horizontal rotation about a vertical axis if using anything other then a Leica/Leitz viewfinder.
I do occasional weddings using Leica M equipment and the last wedding party was so large, I used my Nikon Fm3a with 24mm and flash for group scenes. For the M, I have to resort to a handle mount METZ flash because the Viewfinder takes up the hot shoe and NO ONE makes a double hot shoe anymore. This also negates TTL flash on the M7.
The 21mm f2.8 ASPH Leica lens is sharp with great depth of field, it makes an excellent lens with the M7 for candid work, with zone focus, all one has to do is 'point & shoot'.-Dick