Upgrade my older Linn Sondek or move on?

You mentioned plug and play. The Rega P8 is the definition of that and can  be had in the low $3K range and if you put it on a wall shelf on top of a Townshend platform (I think around $5-600 nowadays since Max passed away unexpectedly) you will eliminate vibrations dramatically and foot falls completely. If foot falls are not a problem, the Townshend alone will solve your vibration problems. You get the RB880 arm which is fantastic. You can't beat the value, and it is in your budget range (including a good value MC cartridge - you didn't mention wanting to replace that or what phono stage you are using).

If you want to spend more and start mixing and matching arms and tables, you can go down the path like @mijostyn and others are mentioning (unless you want to spring for an end game table like an SME) or you can get the Rega P10, but that price is getting into tables with excellent built in suspensions. There are no dealers of SOTA near me and I will not buy a table and ship it. I personally would stay away from VPI because their arms are a rip off.

The LP 12 is legendary, but too finicky and way more expensive than it used to be (what isn't? )new things have come out in the last 50 years. I'm not saying you have to get the latest and greatest, but there have been improvements.

@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.

I was in the same position about 2yrs ago.  I had a late 80's Ittok LVII / Valhalla table and decided to upgrade.  I did a lot of research and concluded that upgrading a Linn LP12 is not a black art, rather it is a carefully considered process.  There are several non Linn upgrades that are just as effective, and maybe more so, and certainly cost much less in direct comparison to official Linn alternatives.

I would upgrade the bearing to at least Cirkus level, and upgrade your arm to at least an Ittok LVII.   

Hercules II outboard power supply.

Mober Inner Platter, Linn outer platter and new springs grommets etc.

Stack Serene chassis, top plate, bottom plate and arm board.

Upgraded DIN to RCA cable. 

Watch several You Tube videos until you can literally play back a complete video in your head from memory.  Then dive in.

The upgrade and building process is not a black art or rocket science.  It is very simply actually, but needs careful patient work.

I was rewarded with a much more expansive sound.  The first thing I heard was a darker, quieter background, which allowed microdetails to be more easily heard.  Speed stability increased.  Bass was deeper and more precise.  Midrange clarity improved, and treble is cleaner overall.

I was able to sell off unneeded parts.   Net cost of upgrades was less than $1800 US, and I now have a table that easily defeats a Majik level build.

I would like to thank everyone for their input. @iopscrl you are so right. There is no black magic here. I jumped in and was able to adjust the suspension and in turn level the arm board much better. Bounce is in agreement with what I’ve read and seen.
I will go for the Circus, Ittok LVII, and Hercules II for the first round. Your thoughts are in line with many others. I do generally like the table. Beat the pants off my old Rega P3 and sounds better than my brothers Thorens TD-160 which he has meticulously set up. Maybe step up the cart from the Nagaoka MP-200 as well. Any suggestions? I need the higher output of a MC due to my vintage Fisher CX400-2 preamp.

Do you mean to say you need the higher output of an MM ( not an MC)? In any case the Nagaoka is a moving iron. I’d move up to MP500.

And why was your Rega wearing pants?