... Why is lack of sharpness on the 500mm not an issue with a 645 negative? Is that due to the increased level of detail compared to 35mm?
I didn't say it wasn't an issue, just not a big problem. A medium format neg doesn't get enlarged as much as a 35mm neg for the same print size, so a slight lack of sharpness on medium format isn't as noticeable as on a 35mm neg. If you enlarged the 645 neg the same amount as a 35mm neg then it is a problem!
Simple solution. Buy a Bronica ETR series and get both the 645 back and the 35mm normal and wide backs. No lens adaptors, full AE with the AEII and AEIII finders, etc. At today's prices for these, it makes sense to go that way. I've used the system for some decades as both my mf and 35mm slr system and have no regrets. A decent kit can be had for $200-300 and lenses are really sharp. The make a 250 and 500mm lens plus a 2x if you need/want more reach.