The concept of a predictable and maybe adjustable force counteracting the pull of the belt has potential imo. The idea of using an old belt sorta feels right too - belt against belt - similar dynamics maybe. Nice one.
As regards measuring the acoustic breakthrough - you may already know this but an old trick is to use the cartridge itself - sensitive transducer. Fit an old stylus or change headshells to an old cartridge - something you are prepared to lose if accidents happen.
There are many variations on this basic experiment but to start just lower the stylus onto an old disc with the motor off. While listening via headphones (speakers off for now) just gently tap around the support area, shelf, rack, plinth etc. Get a feel for the type of sounds produced.
Now take off the belt and repeat this experiment with the motor on - you don't tap now, just listen for motor noise getting through to the record.
With the belt still off (with idler drives remove the platter) place a matchbox on the deck to the side of the platter area so the stylus can be lowered onto it . Turn on the motor and listen to the motor noise breaking through to the deck plate.
Still using the matchbox -
Replace the belt/platter and turn on the motor - see if the noise changes - the main bearing is now in use.
Turn off the motor and do the tap routine - this is the noise getting through to the deck plate from external vibration.
I hope that all makes sense - it did when I thunk it. Apologies if it's as clear as mud. Other/better descriptions may be out there..............
These are a few ways to get a feel for how much noise gets to your deck from it's surroundings and how much noise it makes all by itself.
It helps to work slowly, and take notes.
You can get creative and record the output of the cartridge, sitting on a stationary record, while you play loud music or use power tools/hoovers close to the support structure. Just don't get the drill too close to the sensitive bits.............ouch!
All this trouble just to play records? You must be mad!
Yeah Mama, but that's where the fun is.