Flight training, how do I love thee, let me count the ways
. . . and, there are many.
I instructed at MAPD nine years ago. You indeed have the chance to interview with Mesa Airlines for a regional FO job at 300 hours. Whether you get a real job after interviewing successfully will depend in large measure on Mesa's hiring needs at the time.
Comair in Florida has a similar program. The difference is you must instruct at the school for a while to hit Comair hiring mins for school graduates.
There are tons of other good commercial flight schools without airline connections, such as FlightSafety in Vero Beach, Florida, another of my former employers. I liked that program and will vouch for its quality. You can earn your Commercial-Instrument-Multi in about six months. You really need to earn your instructor ratings, too. An important point about considering a big school is whether it will hire you as an instructor after you graduate.
There are dozens of colleges that offer flight programs. Two well-known examples are Embry-Riddle Aeronautical University (I worked there, too, in Prescott) and the University of North Dakota. One plus about college flight programs is many offer internships with airlines. The great thing about internships apart from learning is the opportunity they offer to make valuable contacts.
You can also earn your ratings at your local FBO. Some offer Part 141 training that is similar to the above programs. Usually, most FBOs train students under Part 61, where your instructor alone is responsible for supervising your training.
Don't forget the military for great training and an almost-guaranteed seat at the majors. But, the military is not for everybody.
Finally, I learned to fly under Part 61 with instructors who owned their own airplanes. My career path was different than most, but maybe similar to yours. I learned to fly because I always loved airplanes and wanted to learn to fly. Later, I decided to change careers and all I needed to do so was to get my multi and MEI. I thought I knew all that was necessary when I got my first job, at Riddle. I was sooo wrong! I was surprised, and embarassed, at how little I knew compared to my colleagues. I studied and got up to speed, but, in looking back, I got minimal training and obtained minimal knowlege during my training. I was working full-time and on overnight shifts, which certainly impeded my progress. If I had it to do over, I would have stopped working, enrolled at Riddle, finished my ratings there, and earned a second B.S., in Aeronautical Science.
Hope that helps. Good luck with your choice(s).