I'm an old man so please take this from a grizzly old guy who has been around the block a couple of times. I just read your post as to you already having a very decent mf camera and your income level. Not sure of your situation but, given these 2 facts is it advisable to start investing in a 2nd system? A camera system can be like getting married, the price of admission may be the least expensive part of the deal. The Yashicamat and most 3.5 Rolleis use Bay I filter systems. This size is more plentiful and less expensive than the II and III but, even a lens hood is not cheap and filters unless you get a series adaptor such as that which is in the classifieds (not mine) also carry sticker shock.

Other than the larger 6x6 format, the tlr will not do much that the 645 will not except have a potentially lower view as you look down vs through the finder but, they make knees for a reason.

Years, well decades ago, I was into multiple systems and trying to build several systems and soon learned the lesson and retreated to a single main system that I built up; a mf with a 35mm back so I did not even need a mf and 35mm system. In the long run I saved a bundle and never missed having multiple systems. Today, I do have a couple but, each was acquired at a time when I could afford my Jag and Volvos and did not dig deep for the accessories but over some 30 years dug through the trash bins of photo stores; most of which either no longer bother putting the stuff out for sale or finally realized the stuff was worth gold.

With your level of income, if your current kit is complete, then I'd look for film, darkroom systems, meter, etc. and expand rather than duplicate.

Sorry, if I sound like a father but I've seen too many persons go too far into debt and done too many bankruptcies for them to make a recommendation and then when learning more facts, keeping quiet.