Well yesterday i handled a Leica R-E and Leicaflex sl2. The R-E was also very laggy in my opinion and certainly compared to te SL2. Great camera the SL2 but (too) heavy for me if it is not the primary camera system.
So i decided that i won't be going the Leica R6 route. Smarter is the Nikon route since i have the glass.
Thinking between the Nikon FM3a and the F3. FM3a is newer and has a brighter viewfinder that also does not black-out with slow lenses. F3 is heavier, feels sturdier, has 100% view and is not mechanical but supposed to be less prone to need service anyway. It does not have the copal shutter (of the FE, Fe2, FM, FM2 and ? FM3a) that seem to give problems after time according to my repair guy. Something with the springs loosing tension and 2nd curtain not closing and being expensive to repair. (sound familiar FE/FM owners?)
From what i read i should be able to have a better and clearer view for wide-open focussing with the FM3a.