The most successful PT's I know only do it part-time or on the side as in a side-job.
The ones who work for a big corporate chain gym...are flat broke and dont make shit. They have a 60/40 rule and when they have no client they dont get paid at all...and the gym manager expect them to act as an actual employee by wiping down equipment, putting weights away etc...most of them quit after 3-4 months.
Your best bet IMO is to stay in top shape...convert a basement or a garage or something into a studio. Have the most basic equipment and land one or two clients...get them in shape and watch the roll-over effect happen if you are lucky. Dont quit your day-job...use it as a part time hustle.
I have a buddy who has a large spare-bedroom w/ a couple becnhes, dumbbell set, power rack and various other pieces...he has mostly female clients...he charges $40 a session cash...pay as they go. 4-5 sessions a week translates to about $200 extra bucks a week. His mindset is to use PTing as a way where he never has to dip into his real paycheck from his day-job to pay for gas, groceries, supps, lunch-money and mild entertainment cash. Thats a good mindset to have going into this...then who knows...it might roll-over to where you dont have to have a real day-job but then again...it isnt definite and probably isnt worth risking it. Good luck!