Opinion on what best to do about my teeth/bonding with a fixed retainer? (Photos)

I would first explore the possibility of removing bonded retainer and Invisalign treatment to close the space. Spaces always have a tendency to open so you will most likely need another bonded retainer as well as a removable retainer that you wear nightly to keep it closed.
As far as shortening your 2 front teeth, this is very easy and not a problem.

You are a very good candidate for composite bonding to close the gap--the two front teeth can be reshaped as well to not look so large and square--Also I would recommend widening the upper arch--this can be accomplished with orthodontics, porcelain veneers or more composite bonding--that would give you a truly amazing outcome

Absolutely bonding can be done with a fixed retainer in place! In your case the gap is quite small so you can probably achieve a nice result with only addressing the two front teeth, but my recommendation would be to apply a small amount of dental bonding to the side teeth as well to improve symmetry and balance out the proportions a bit so you don't need to file the front teeth down. Dental bonding is all about preserving tooth structure and applying it in very specific ways can achieve some pretty dramatic results! I would want to make sure that the fixed retainer is in fact as effective as we want it to be as gaps shouldn't be appearing when it's in place following ortho, so in a case like yours you may require an additional retainer after bonding just in case.