Nice that you have original records, from 70’s and 80’s, etc.
Our own 70’s albums are mostly Yes, Rush, Led Zeppelin, Genesis, Fleetwood Mac, et al.
Your music listening sounds like our house. Streaming (and CD’s) does the heavy lifting. Always on and playing. A record player is an occasional treat and more special experience, etc.
Thinking about a desire for parity with your other source. Maybe listen to a few LP12’s if you can. Most LP12 stockists can help with this. Maybe a Selekt level and a Klimax level, to compare and contrast with your ND555 and see how you feel ?
Make a few decisions with your dealer on the basis of your findings…
FWIW, the LP12 Selekt level is a pretty decent turntable.
I would argue at least as good as NDX2. (Sorry, can’t say for ND555)?
The Cirkus is a decent bearing. Karousel is better.
Any external psu will improve on the standard Valhalla fitted internally.
The Lingo4 is excellent, probably the sweet spot in the Linn psu options.
Those two items, by themselves, probably constitute the best additions and a great start.
This should show you an obvious uplift, from stock or basic LP12.
Addition of Kore, Trampolin2 and T-Kable gets you to full Selekt level inside.
Be prepared for one or two extra bits, (not always obvious, such as a cross brace, or arm board), to migrate from standard upwards to Selekt level.
Being a purist, have a bias for the original Ittok (or maybe Ekos from same era). Both are fine tonearms and wouldn’t be concerned to quickly change to more recent tonearms. However, any Ittok is 30+ years old now. It is worth examining the Ittok bearings. Lots of YouTube videos showing how a “swing test” works. Again, your dealer might help here too. It is possible to bring an Ittok (or Ekos) back to full specification with a service and bearing change.
Cartridge is a subject open to personal taste.
The forum search function is your friend.
Also - in time - think about a decent phono stage. Lots of choices here…
Dynavector P75, especially if you select a Dyna’ cartridge. Rega Aria is good value. Linn Uphorik is great for synergy. With a 552, I would be tempted towards a SuperLine.
Advice and kind words from @Bevo is all about right.
IMHO, Selekt can be achieved for reasonable money. Particularly with your start point.
Anything higher gets more expensive, (even using pre-loved parts ) and “law of diminishing returns” kicks in.
Good luck with your LP12 journey.
Hope you enjoy yours as much as ours…
KR
R