That’s actually a very good way to mitigate vibration by placing more mass in contact with the device.
However, vibration is insidious in that harmonics are tough to address. It’s like hitting a tuning fork. If you hit the right frequency, any object will resonate.
There is an engineered material that goes by the brand name Sorbothane. It is a gel like substance that is like slightly cured silicone. It can be found in many things such as shoe inserts. It only comes in dark blue or black but it’s used in industry wherever vibration and harmonics need to be dampened.
The sheets you used are great for sound deadening at higher frequencies. However, you may want to try Sorbothane and see if it will address the other frequencies. That and the slab of concrete mentioned above will help.
Here’s an Amazon search term. for odd shapes it can be really pricey. By odd I mean a 1.5" domed piece can be as much as $30 each.
I might recommend fabricating your own isolation dampeners that install where the vibration dampening feet are located at the bottom of the P1. Then use the strip material so you can vary both the thickness and length. You’ll have to experiment with the length to get the right harmonics.
https://www.amazon.com/Isolate-Sorbothane-Strip-91-4cm-0-318cm/dp/B00B84FNBS/
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Underneath the P1 is a set of holes that have feet inserts.
Bambu sells anti-vibration feet. THEY DO NOT WORK!! But I include them here in case you want to try them. I found them to be useless. However, if one wanted to mount a Sorbothane vibration dampener, this would be a logical attachment point. You would of course have to fabricate an adapter, but hey, you have a 3D printer for that. 
They’re only $11 plus $30 for shipping. 
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There is also a design on Printables that also uses TPU filament as a vibration dampener. I do not see TPU as being viable given it’s rigid nature. But the designer did give you a head start if you wanted to use his brackets as a basis for fabricating your own dampeners.
https://www.printables.com/model/607838-space-saving-anti-vibration-feet-for-bambu-lab-px-/files