Upgrade my older Linn Sondek or move on?

Yes, MM. Too much Sauvignon Blanc tonight.

Unfortunately, I believe in order to get the best out of your LP12, it does need a precise and correct set-up. If one is willing to do the set up oneself and can accept the fact that the set up might be at the 80% level ( at best), then the right path for the OP is to do upgrades. The best bang for the buck, IME, is the Karousel bearing, which is at the heart of the table design. If one can upgrade to the Karousel and can utilize a majik subchassis, I think the OP would be off to a great start.

@thermionicemission I am visiting and reading this thread with a interest, as I own a Linn TT, which is a little younger a model that your own. It is a TT that has not had a regular use for many many years.

I am not a Engineer but have had a lot of experiences of having works undertaken on certain functions of a TT. The Bearing Assembly has been an area of interest for numerous years, where the rigidity of the bearing housing when in place has been worked on and the Internals of the Bearing have been worked on. All examples of work produced to date have been quite successful as the outcome.

@daveyf has referred to the Karousel Bearing as well as the Majik Sub Chassis.

The Bearing I am sure will benefit from a particular type of treatment or a replacement might be the easier option to achieve a very similar end product.

The Majik Sub Chassis is solely a device to create a rigidity and decrease a deflection in the chassis, which should then ensure the bearing housing is not impacted on by the deflections, with the result being the axis of the Platters Bearing Spindle is truer in rotations. With less energy being produced as a result of a decrease in deflection, there is also the likelihood noise is reduced as well as less mechanical movement is present.

Within this Forum, it is no secret I am a advocate of the use as chassis material produced from a Resin Impregnated Densified Wood such as the Brand Panzerholz or Permali. Such materials are also used by other TT producers and have been a material used by the savvy forum members for many many years. I am a Greenhorn to the usage, and will not be returning to earlier used materials. 

It is not too expensive or difficult to acquire a Permali Board at a dimension that would suit the role as a Sub-Plinth / Sub-Chassis.

Linn in their latest offering of a TT with a value of approx’ £50K are making it known they have now adopted a Resin Impregnated Densified Wood, they are using a term Bed-Rok, as a disguised name to identify the source of the material. 

I am sure there will be third party versions of a Bed-Rok upgrade to be seen very soon, that is a no brainer, and Permali or Panzerholz will become much closer to being a Household name for a material. 

It might be prudent to wait and learn if the Densified Wood is an easy option to be produced as a mimic for the the latest Linn TT, and jump forward a few generations in relation to upgrade designs if achievable.

The following will show where my info about Linn is sourced from.

Linn was already in the process of incorporating a new wood technology for its plinth (the case), which pushes layers of timber together under high pressure to create a facade that is acoustically silent. (Whereas an acoustic guitar or other wooden instrument uses its wood for resonance, a turntable wants to mute those vibrations.) LoveFrom spent hours on calls learning the boundaries of the design, and homed in on dozens of details that it could address in the product.

https://hifiplus.com/articles/linn-sondek-lp12-50/

If this is a adopted approach, this methodology will have the potential to put the TT under discussion, at a place where it is able to out perform mechanically suggestions for alternate TT's  already referred to in this thread, especially those dependant on Aluminium to control transferred energies.

 https://www.lessloss.com/page.html?id=80