You need to get your billable hours up and they only way to do that is to have more manpower. You cannot afford to turn away work. People are too busy these days and if you are not accommodating they won't come back.
Part time techs are an option. When I was expanding I hired “B” rated techs part time with the opportunity for full time as the business grew. I also paid them a wage that was lower than they would expect, but paid them commission based on productivity. I found that techs like this. In a busy week they can make money.
As our client base grew, I put them on full time.
In my opinion, the most important thing you need to do now is concentrate on getting your car counts up. And this requires not turning away work.
Another thing, your manager needs to manage. That's the way you make money. Keep him out of the bays.
I struggled with the same thing. Trust me, do this first.