Given your preferences, I would consider buying an RP8 if I were you and selling off both of your decks.
I think that you will find that if you have purchased the Groovetracer RP6 sub-platter that you cannot fit the Delrin platter.
The Delrin platter is a significant upgrade in my opinion. I have three Regas with Groovetracer reference sub-platters and Delrin platters and an acrylic platter too.
A tonearm upgrade is a significant improvement too but you would want to consider an RB-700, RB-808, RB-900 or RB-1000 etc to achieve a significant step up in performance over the RB-303 on your deck. The alternative would be an Audiomods arm.
Have a listen to an RP9 or RP10 to see if you like the sound with the ceramic platter. One of these might be in scope if you sell off the other decks.
Get a decent cart and phono stage.