You can have a single-pilot operation and management and have a different pilot be the single-pilot listed in your ops specs. For what you are trying to do you may be better off getting a LOA for aerial scenic tours. This means you can fly people for sightseeing w/in 25nm of departure airport for hire. The benefit of doing this over a 135 is that it is all legal under part 91 flight rules. The only thing you will have to in addition to what you do now is probably enroll in a drug and alcohol program.

A 135 would probably have way to much overhead for your limited operation. The huge kicker with 135 is gonna be the insurance on the aircraft. Not sure what type of plane you have but flight instruction insurance vs 135 insurance is quite different.