You have a nicely shaped head, striking features and large eyes. If you get rid of the thinning hair people will focus in on your eyes and not on your receding hairline.
I think that you should definitely go for both the shave and the proposal this weekend. If your GF says "Yes" she takes you as a packaged deal - and not for whether or not you are losing your hair!
Just remember the 30 day rule. You have to give yourself 30 days to make the mental adjustment to your new self.
Although I have a beard myself, I don't think that you need to grow one. If I had a nice strong chin like yours I wouldn't hide it. I think that you look good clean shaven, and just a bit "dangerous" with a few days of stubble!
Good luck this weekend and let us know what she says