I’ve never like the cables, especially on the large windows. I have seen the cables so stessed that the begin to eat through the wood as you tighten them or load them.
I understand your method and can only say that it sounds like it will work but it is not what i would do.
Can you jack up the window (very slowly) from the outside and put knee braces under it where the windows mull together? This would require good blocking at the points where the kne braces contact the window and the dwelling.
Personally, I have yet to see a bay window, built or bought that hasn’t sagged at least a little. Unless it was built on cantilevered floor joists.