Since you have living space above the garage, I would strongly suggest you keep the insulation installed. There is a reason the previous own installed it, and that is most likely inadequate insulation between the garage and living space. You will have increased comfort and reduced energy bills by keeping the insulation.

The actual question you need answered is whether the spring is designed to work with that door. Garage doors are engineered systems that carefully balance the spring force to equalize the door weight as it is lowered. A well designed system does not need a powerful opener. If the last owner replaced those panels, it is not balanced and needs to be corrected.

Tightening the spring is a stop-gap measure, which hopefully they explained to you. It will help the motor work when the door is mostly lowered, but it will in turn cause too much return force when it is raised. There is no balance. You can install a more powerful motor to try to brute force it, but even that will get more wear and tear than it should, leading to more noise and reduced service life.

Furthermore, motors are cheap. It makes zero sense to replace panels at around ~$200 each, when you can replace the motor for less than a single panel. Not only that, you're paying more to downgrade your garage. It's .. not sensical. I don't know what set you on that path, but you really should reconsider.

What you need to do is find a legit garage company that will come out to spec and replace the spring with the correct gauge and number of turns. If, when properly balanced, you still need a bit more oomph on the opener, that's trivial to do. That can be a DIY job because changing that isn't dangerous. Changing the spring should be done by a pro, though, and if they're out there anyway, they should give you a good deal on swapping the motor.

You have not been advised well. That may be due to their incompetence, or your misunderstanding, but consider finding another garage door service.