Primarily better tonearm - new, much higher quality bearings. The better the tonearm the more you get out of your cartridge. I am familiar with the 505 - I was a high end distributor for a few years and personally owned some of the micro TT's and arms including the 505. Very good arm, but everything wears out, particularly bearings.
The early DD micros are quite grainy sounding to my ears. If you prefer DD then get a new one with more advanced speed control systems than what was used on the 70's & 80's.
Basically a $500 cartridge on a top level TT will outperform a top level cartridge on an average TT. doesn't have to be a Rega - I just suggested that as a starting point. There are plenty of others such as SOTA, Feickert, Kuzma at various price points.
From 30 years of distributing high end audio and setting up 100's of high end TT's my advice - start with a better TT. Both of your existing cartridges, unless they are worn out, are capable of much more. I've also owned and or set up both of your existing cartridges.
The ZYX, if it has a family sound tends to neutral and resolving. On of my personal favourites is the Omega, it does sound a little fuller through the midrange compared to models above and below.
Why don't you find a dealer and ask them to demonstrate a very good TT ( $4k up ) with one of your cartridges mounted - the Shelter would be best since the Koetsu is less neutral. Find out for yourself.