I've got a Sharp with the same problem, bad LCD display unit.
Like you said they want way too much for a replacement. In my case
nearly 50 bucks just for the crystal module.
all else works fine on the oven, you just have to push the buttons
carefully to get whatever time and cooking level desired.( I'll find a
donor some day with a dead magnetron that'll hopefully have a good
display. Untill then it's push the buttons slowly and carefully.)

Is there room to drill a hole in the board on each side of where the
ribbon connects? If so you could cut a little block of aluminum, file it
so it has a ridge down the middle, then thread holes in the ends and
screw it to the board so the ridge presses on the ribbon and holds it to
the contacts.