What is the ideal weight/mass for a turntable?

I am sure there is much research on the subject. My opinion is just from a layman's point of view. It looks like the most successful designs are ones that incorporate heavy massive structures with substantial and extremely heavy layered platters.

I don't know if there is an "ideal" weight. But if there is one it is significantly more than the weight of your P9. ;-) Just kidding, slightly.

Why would you want to make a tt any heavier than a P9?

Is it not a very elegant solution to the task of spinning a vinyl disc?

There is no ideal mass and in and of itself a turntables weight is meaningless. The proper mass can only be discussed within the context of an overall turntable design with the critical factor being the type of suspension, if any. You can make an excellent low mass turntable, but the cutting edge designs all seem to be quite massive. That could mean high mass is necessary or it could also mean that it's easier to design and build a high mass turntable than a low mass design of equal sonic merit.

Rega is all about low mass, high rigidity. That's one "very elegant" way to go. It is effective in many ways up to a point. IMHO, ultimately, high mass will prevail. (see my system)