I'm also an RF645 fan (have two, the most recent of which I bought because it came with a 45mm lens and a polarizer filter... ;-). I've also owned the Ikon, and currently own an M2.
Most of my photography is in two different areas: (1) daytime portraits of people, cities, and landscapes, and (2) low-light/contrasty pictures of performances (circus, cabaret, vaudeville) in small clubs. The RF645 is wonderful for the former, but not so good for the latter, as you may have found. I've shot many rolls of MF Delta 3200 (usually at 1600, always dev'ed in Microphen), at f/4 @ 1/125, and can just *barely* get useful shots. When I get a good one, it's excellent: lots of tone, relatively little grain. But that route has ultimately been frustrating: f/4 (or 4.5, with the 100mm) is just not enough light, and not enough flexibility.
As you've posited, 35mm RFs work wonderfully in low light, and if you have a fast lens, you should be able to shoot with some degree of adjustability, rather than being at the extremes, as I often find myself with the RF645. I'm currently using a Voigtlander 75/2.5 for most of this work, and it's alright, although I've realized that a faster 50mm would be more useful -- in fact, I've just ordered the Zeiss 50/1.4 Sonnar, and am looking forward to using it.
I wasn't all that pleased with the Ikon. It shot okay, but is not built nearly as nicely as the Bronica, nor a Leica. The meter is fine in daylight, but jumpy; I found it was much easier for low-light subjects just to use the external spot meter and set the Ikon manually, occasionally flipping shutter speed to allow for brighter/dimmer light. The Bronica's meter is fantastic, and you're probably spoiled by it, as I was. The viewfinder of the Ikon is bright -- like your Bronica -- but the readout is not nearly as nice. Instead of the clear side-display of those green LCDs, you have very bright red shutter numbers, super-imposed on the image. I found them distracting, when I could even see them (I wear glasses).
I sold the Ikon a year ago, but recently dove back into 35mm, first through an M4P (too many framelines, and felt a bit chintzy), then an M6TTL (whose meter seemed to fail immediately), and now to a 1950s-era M2, which is wonderful. I carry a meter, which really isn't a big deal, and is freeing in some ways. (Granted, I learned on an old Rolleiflex, also w/o built-in metering.)
So there are my observations. A final observation I'll leave you with is that I've realized that it's always a problem for me when I have two cameras that are too similar. If I have multiple cameras, I've learned that they should have radically different looks and feels (both in shooting & imaging). At the moment, anyway, I'm very much enjoying the M2 and its mechanical nature. I know I'll pick up the Bronica for something a bit slower and less everyday, and the Rolleiflex for something even slower. All have their purposes, and individual personalities.
Hope this helps.