Upgrade my older Linn Sondek or move on?

MOVE ON!! But, no to the well Tempered or VPI. A isolation suspension is crucial for the best performance. The Kuzma Ref 2 or the Stabi R on a Vibraplane, The SMEs, The Basis tables, The Avid Acutus then the best value of them all, the Sotas. You can put any turntable on a vibraplane or MinusK, but you will save money getting the suspension built in. The Sota design borrowed by Basis, SME and Acutus is far more stable than what you have in the LP12. I do this demonstration all the time. Put a seismograph application on your phone. Put it on the surface your turntable sits on then jump on the floor and bang the surface and watch the response on the phone. Now put the phone on your platter and do the same. On the Sota Cosmos platter you get absolutely nothing. Not even the tiniest squiggle.  People think if they put their turntable on a big heavy object that will protect them from environmental rumble and noise. That application will show you that is nonsense.  With the system like yours I would get a Sota Sapphire and put the best Rega arm you can afford on it. The RB2000 would be killer. Sota will mount it for you. Get the dust cover. You will be blown away by the improvement and ease of handling. Do the seismograph test and smile!

@tvrgeek 

Boy, you are not kidding. I owned two LP12s in succession. It was the best reasonably priced turntable in it's day, 1970s. But it was a PITA to live with and I rejoiced along with the HiFi press when David Fletcher released the SOTA Sapphire. It was the very first suspension turntable that was a joy to live with.

@overthemoon 

Sota will sell directly to you if you do not have a dealer near you. They will mount the arm and cartridge for you. It will be shipped ready to go with only one exception. The heavy platter will be shipped in it's own box. All you have to do is slide it on the bearing then put on the belt and motor cover. You will also have to adjust the feet to level the turntable and plug it in.