I found that shooting 645 was not a benefit. I used a Rolleiflex for a few years, and loved it. Perfect camera. Then I wanted interchangeable lenses, and I was gifted a Mamiya 645. I loved especially the 150 lens, but using it for portraits was borderline impossible, or impractical at best. So I got a Hasselblad and three lenses. Now I kind of wish I had that Rolleiflex again...
The grass isn't greener on the other side, but in my experience, nobody learns that lesson without going there to check for themselves. You already know you like the Yashica, and that cropping the 6x6 negatives is a piece of cake. If you want to use 645 I would recommend either loaning a 645 camera from someone you know, or get a cheap Holga and put the 645 mask in it. Then see how you enjoy the 645 format after printing a few dozen of them.
Finally, I find it difficult to take negatives from my Rolleiflex days and combining them with negatives from the 645 days and make coherent looking series of prints from them. Usually I use most of the area of the negatives, regardless of format, so I end up with some prints rectangular and others square, and that does bother me quite a bit, actually. Just throwing that out there in case that is something you had not considered.
Good luck with your choices.