One tidbit to add about scopes: some biological scopes do use "epi" illumination like metallurgical scopes. It's commonly used on high end scopes, in combination with laser illumination and fluorescence. They are much more complicated and require much better alignment than a regular trans illumination scope.

I suppose you might be able to get slightly better resolution using a shorter wavelength, but at that point, it requires a lot of technical skill and environmental conditions and time and money, Just getting to the point you've reached (and knowing what the limitations are) can be satisfying in itself.