upgrade path

Check out the Basis 2001 with the Graham 2.0 on AgoN that would be a great next table. The Graham arm is very adjustable, you'd learn cartridge set up and adjustment and the Basis 2001 with its suspension isn't as susceptible to vibrations so isolation is not as difficult as with some tables. You have two very good components together, and he's open to reasonable offers!!!

vpi classic is the best way to go. The vpi can then be upgraded with the ring and other tweaks. Great sound and value.

The 1200's a great table. Think about upgrading your phono preamp and cartridge.

You have to do some search and then decide for the right mods. I mean real upgrades (bearing, electronics, plinth) and then look for a copper mat, a peripheral ring & stabiliser.
Are you shure that you can mount the SME 309 on this armboard?
You must first measure the pivot to spindle distance & then you have to find the corresponding effective length. Then you can search for arms with exactly this eff. length. Also, take care for the proper match between your current armboard & the potential arm base (of the new arm).
If you can avoid the fixed arm base and you can construct a new (outside the chassis) on a heavy surrounding plinth, then I think it is better to look for the SME 312S.
I'm warning you: The transference to a belt drive is a very traumatic experience that you are going to regret.
Leave the luxury marketing objects to some fashion victims & proceed carefully with what you have.
Trade your AT440 for the best AT you can afford and stay to MM, otherwise you must have very deep pockets for equal quality MC & a worthy phonostage (very few in the market).
Good luck

George

Samhar made the best recommendation