it's very possible that a simple solution would work, provided the situation is really as you describe it. An orthodontist told you differently, so I have to wonder, but it also sounds like he just gave you a routine time estimate for his full treatment case without really considering a limited treatment plan.
A few pictures would help.
Sometimes our goals are more limited than the "complete" or "comprehensive" option presented to us. While a full set of braces is "best" and more comprehensive, sometimes the time or money involved is more than we value or are willing to commit. Options are nice. Often in cases like this, a spring retainer, or better yet an Inman aligner, is a better option.