Once you have your replacement mirrors,you might consider applying several coats of varnish to the silvered face which extends slightly around the ground edge. Theorectically this should slow down water ingress.
The issue will be whether the varnish attacks the silvering, so may be a non-solvent-based varnish is best.
It is not something I’ve tried, my mirror still has the shabby chic look that people pay thousands for.