Steve, you're right; the most important thing is being safe and having fun.
And I think you have half of the weighting principles right. If you can hold a stop at 15 feet with 500 psi in the tank, you are not UNDERweighted. You may still be grossly OVERweighted, though. It's pretty easy to find out. If you bleed the tank down to 500 psi, and stand in water about chest-deep, see if, with all the air out of your BC and your breath fully exhaled, you can lie on the bottom. If you can, and still float upward with a deep breath, you are correctly weighted. If you can sink without exhaling, try taking some weight off until you can just sink when you have exhaled.
Then you can arrange the amount of weight you've determined you need to adjust your trim.
It really isn't important to fine tune this to the nearest 6 ounces
But carrying down 8 or 9 pounds you don't need is costing you a great deal of efficiency, because you have to carry air in the BC to compensate for the overweighting. From your original post, it's clear you're thinking about streamlining and efficient movement underwater, and if that's one of your goals, getting your weighting close to right is a big step toward it.
I hear you about being happy with the weight distribution you've found, and how it works to balance you underwater. But if you buy a BP/W system, you're going to have to redo your weights anyway, because you will lose some positive buoyancy when you switch from the BC, and you'll gain some negative from the backplate. So why not begin the process now? That will help you decide more precisely which wing you'll be happy with in the long term.
And I think you have half of the weighting principles right. If you can hold a stop at 15 feet with 500 psi in the tank, you are not UNDERweighted. You may still be grossly OVERweighted, though. It's pretty easy to find out. If you bleed the tank down to 500 psi, and stand in water about chest-deep, see if, with all the air out of your BC and your breath fully exhaled, you can lie on the bottom. If you can, and still float upward with a deep breath, you are correctly weighted. If you can sink without exhaling, try taking some weight off until you can just sink when you have exhaled.
Then you can arrange the amount of weight you've determined you need to adjust your trim.
It really isn't important to fine tune this to the nearest 6 ounces

I hear you about being happy with the weight distribution you've found, and how it works to balance you underwater. But if you buy a BP/W system, you're going to have to redo your weights anyway, because you will lose some positive buoyancy when you switch from the BC, and you'll gain some negative from the backplate. So why not begin the process now? That will help you decide more precisely which wing you'll be happy with in the long term.