Best type of metal for turntable platform?

YES! I have both of those issues, especially footfall. One because of the flexible floor and the other from surrounding vibrations. I have to tiptoe to avoid woofer flutter. and because of limited space I have to keep the whole setup in this area. I was ultimately never able to use a subwoofer with the turntable because the floor is so sensitive. I don't use a sub with 2 channel but sometimes I want to crank of the bass. Cannot do wall mount in this area. That was my first choice.

  If I were to machine a plinth, I’d choose Corian, rather than any metal.  

I would use the Panzerholtz, I prefer it of all.

Yes, I live in an old home, with ‘flexible’ floors as well. Had similar issues, and tried all types of things (which worked to some degree or another) before I got my SOTA Sapphire.

As I always say now, get a SOTA, problem solved 😉

But, first off, is your TT on the first floor with a basement below? Is the basement unfinished below the turntable? If so, a cheap way to add rigidity to the floor is to get a few 2x4’s, cut them to slightly more than from the top of your basement slab and bottom of your floor joists, locate then under the turntable area, and along a couple of floor joist spans, and use the 2x4’s to reinforce the area by simply hammering then tightly in place. If the 2x4’s bow a little, that’s fine, that means there is tension. No need to nail them to anything, simply friction fit.

Yea, technically you should use treated limber, but 2x4’s are cheap, so if they ever start deteriorating against the concrete, replace them. Unlikely to happen unless they are there for years.

You might still have to do more, but that is a great start if you can do it.

There's crawl space under my floor. SOTA has came up many times for my situation, but I do really like this TT and want to make it work. Even if I wall mount I feel that the flexible floor vibration will creep into the wall.

I even replaced the bottom of my whole BDI audio rack with 2 types antivibration pads (one rubber and one with cork) that are for washer/dryers and it really didnt do anything.