Stanchions like the ones you had fabricated are common on larger yachts and seem to do what you intended them to do, awesome, and nicely done.
An old school trick to reduce vibration transmission is to fill the stanchions with sand, sounds weird but really works.
As mentioned, Sylomer vibration isolation foam of the correct density (the grey or red versions) would also reduce vibration transmission. Comes 1/2" thick but you can trim the sides as long as you keep 1/2" in the load bearing part of the device. Any poly adhesive works with it.
As has been mentioned, isolation mounts are also common in this application. The mount typically used is similar to these BRB mounts Captive mount BRB | AMC Mecanocaucho
which Soundown carries and provide positive capture so no worries with movement. The Grainger isolator is not typically used because if it deteriorates the equipment may come adrift.
Use an isolator only at one end, not both. This avoids potential resonance.
I have the Soundown Carpet underlayment in my own boat and it really works with airborne noise. I doubt it would do much for vibration, but it does feel really soft underfoot.