Sticking with what you have is always a great idea. I'm trying hard to do that too, but with only moderate success.
And in spite of offering you a 5x7 for sale, I was looking at what you said you want to do and kind of think your idea of an 8x10 camera makes sense. You can put a 5x7 or 4x5 reducing back on it, and generally, you'll end up with considerably more bellows draw, which you very well might need for portraits. Especially true if you've got some long lenses to mess with. Plus 8x10 is a wonderful format itself.
Usually, it shouldn't be hard to make a reducing back if you already have a working back in that format. It is mostly a flat board with a hole in it. Then just screw your old back to that. That is basically what the lovely Deardorff reducing backs are. You've got a 5x7 back from you B&J that could be a donor if some fabulous deal of 8x10 2D comes along.