I have upgraded the bearing for my Technics SL1200 MKII turntable and then removed the plastic cover that is above the circuit board. If one look at the inside the TT there is some room available:
Would it be an idea to have some kind of resonance absorbing components in the plinth or would that be futile? F.ex. dampening of the circuit board in some way?
I just hate it when people say "Dampeneneneneing." To dampen is to ing. Dampen and Damping will do just fine.
Get some peel and stick variety of damping sheet material. You can cut it with scissors and stick it wherever you need it. PartsExpress sells a very affordable generic version, otherwise go and buy some Dynamat.