Tonearm mount to the plinth vs arm board vs rotating arm board vs isolated tower

Keeping the two parts still relative to each other isn’t easy.

Perhaps not.......
But certainly not impossible 🤗

Amateur?    Hmm. 
I would point you to this post.....

 OMA (Oswalds Mill Audio) Debuts K3 Direct Drive Turntable—An AnalogPlanet Exclusive Michael Fremer  |  Feb 28, 2020

The OP in this thread, is asking about an idler.

@kanchi647
The idlers are one of the crudest, oldest form of turntable, and with them comes.....(based on my personal direct experiences)

The most vibrations, least isolation, the most noise.

That's what you are dealing with.

For these reasons I would recommend you try a design that works to eliminate those three (at least) design symptoms.

When done well they (idlers) can be quite good. I own a Jean Nantais Lenco that contains reference level (Parts) see my virtual system. His design was called the best turntable by blogger A. Salvatore at one time.

His armboard design specific to your thread question, has gone to great extent to reduce the symptoms mentioned above.

To give you and others reading an idea of how far he goes; here is a picture of mine showing the extent he goes to....to reduce vibrations, noise and provide for isolation. 

https://photos.app.goo.gl/MdYZd7sBGJ1cCHyC8

Similar cavities exist under the motor. Even with these huge cavities, it still weighs 100 lbs.

Those who intend to build an new plinth for their TT's are in different
position from those who want to keep their TT as is. My Kuzm Stabi
Reference accept only 9'' tonearms. Triplanar can be mounted
because the arm(wand) is on the side of the VTA tower allowing
for longer lengt. I wanted an second arm and was forced to use
an armpod + 12'' arm. I ordered both by Reed. My Kuzma and
armpod rest on aan sand filled base resting on 3 spikes 
in an professional rack ( German ''Copulare'') . I have no idea
what ''optimal solution'' means but am satisfy with the situatiom
''as is''. 

Halcro, if your arguments are good, it’s really not necessary to be insulting.