I was in your exact situation at one point. I had no interest in the regionals or the airlines and really wanted to aim for part 91 or 135. When the time came to move on the only place really to go were the regionals or piston freight. I figured I would just fly part 121 until my dream part 91 job came along...that never happened.
Ive grown used to the airlines and for me it is all in what you make of it. I will not leave my current airline until I have no choice. I would advise that you keep networking (never stop networking) and take a job moving up. Something multi and preferably turbine. As soon as you hit 1500 hours get your ATP and you will become more marketable at jobs outside of the airlines. You may find that the upward step you are looking for is only one choice..the regionals. Find a place you can tolerate to hang your hat for a bit and keep your eye open for your flying dream job.