What is the ideal weight/mass for a turntable?

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)

I think the the issue of turntable mass is largely misunderstood. The more important issues are rigidity, damping and rotation stability. Mass has little if any direct impact on these key issues.

Rigidity is very important and it often requires a lot of mass to get it. A good milling machine weighs at least a half ton because it needs to be very rigid. The most cost effective way to get rigidity with a mill is to use lots of steel and iron. You can get the same rigidity with lighter
materials but it would just raise the cost with no benefit. With turntables there are a lot of ways to get rigidity and the most cost effective ways tend to also be heavy. So rigidity is the goal and mass is just a side effect. Heavy turntables will often have superior rigidity, but this is not always the case. For example a carbon fiber base will lighter and more rigid than one of the same dimensions made from aluminum.

Damping is also frequently misunderstood in it's relation to mass. Adding mass to a turntable (or any structure) can not by itself increase damping. The argument of high mass being more difficult to excite is bogus. A 150 pound chunk of granite rings like bell. It take no less energy to get 150 pounds of granite to ring than it does a wind chime. However, like rigidity, effective damping techniques are often also heavy. Sand and lead shot are very effective
and simple ways to damp resonance. It is not the mass that helps it's the damping. So once again high mass is just a side effect of some of the better methods for damping.

Rotational stability is certainly affected by mass but there is much more to the story than meets the eye. Mass mostly effects the frequency of speed perturbations but it does not make speed more constant in absolute terms. A heavy platter responds to a perturbation with a small speed variation but it distributes that variation over longer period of time. Conversely a light platter will have larger speed variations but the recovery will be quicker. Most, but not all, prefer the resulting sound of a heavy platter.

So in the end high mass turntables will often be superior to their lighter weight peers. But if so it will be because of better rigidity, damping and/or rotational stability, and not because they are "heavy".