Shock mounts check out okay with no play, anti-sway bar bushings check out with no play however they are dry as far as lube goes.

 

When dry, they will squeak, but not thump. I'd try getting under the car with a 17mm (or is it a 19mm?) and try tightening the LCA nuts to see if any move.

 

So this thing is still happening.. left bearing was replaced.. still happening. I'm getting buzzing while decelerating so i'm suspecting something with the exhaust.. all bolts are as tight as i can get them. I'm suspecting 2 problems.. one of them with the exhaust, the other with the springs..

The thumping could also be the helper springs collapsing fully because it mainly happens when the front end is loaded a tiny bit but not at max load (hard rights while braking).

Helper springs are not fully compressed at rest.

 

Not sure if your issues are mounts or not, but this is the experience I had:

Stock mounts at 170k. Vibrating metallic sound that convinced me it was my exhaust rubbing somewhere. Exhaust shop reccommended i look at mounts after describing the issue. Occurred under deceleration and making right turns (if I recall, and mostly at slow speeds).

Both of my mounts were completely shot and had separated. Replaced with the competition mounts, and could not have been happier (excluding this https://www.miataturbo.net/diy-turbo-discussion-14/turbo-ruined-56181/). NVH increase is minimal and acceptable for a street car.