Well, your idea is a bit crude to be honest.

The first thing that you don't really seems to be 100% sure how you want to handle special attacks. Yes, the special attacks on SoM are incredibly slow, specially when you want to use a high level attack (charge to 100% 8 times? really?), so I would say that the the SD3 approach is much better, I would go with that.

I also think you could add some gameplay to the special gauge, for instance some character could gain a bonus gauge when it hits a critical hit, one of the saber spells could add bonus gauge and, finally, a tanky char could earn points by getting hit.

As for spells, I don't really know how to handle it. When I played I had two problems, MP is boring and spells are either too powerful or too weak. In a quick brainstorm the only idea I could come with was that stronger spells (such as saint beam) should require gauge to consume, while weaker ones could be spamed.

Finally, a free for all weapon choices is great on paper, but it is far too much work and very small reward. Specially because in the end, one weapon will be better than others and people will simply ignore all the work you put on the other weapons. If you really want, focus two or three play styles per class. You could take a look on Soma bringer, they handled it quite well IMO.