Well you said some would beg to differ...I beg.
If you are comfortable doing 6 or more high speed rolls across the sky and you KNOW when you release the sticks which way the canopy is going to be pointing, you (and the plane) are probably good enough to learn 3D.
I wouldn't count on that. I have planes that will roll on a string, fast enough to make counting them difficult...but they don't neccesarily make good 3 dr's.
CoG - generally as far back as it will go, but you do still want the plane to fly well
CG is very important too...but I don't think as far back as it will go is going to help you do anything except crash. Tail heavy planes don't fly well...or long!
My advice is this...get yourself a plane that has a design that lends itself to 3d flying. Practice a lot on the sim...and at the field. Just make sure you go to a nice high altitude...3 mistakes high...and do your thing. Start out with hovering. Once you get the timing down with the sticks and have good control, then you can move into other "high alpha" moves. Harriers, rolling harriers, etc.
Set up your planes on the sim to have as much throw as possible on ALL surfaces. Lots of expo will tame the sensitivity on the sticks. When you can handle your plane on the sim, without crashing, and are satisfied you can make the plane...not the canopy...do as you desire, then and only then are you getting warmed up to actually try to fly 3d.
Good luck... and keep asking questions here, and at the field, and before too long you will surprise yourself
.
Jim C