Moved from a Lyra Kleos to a Miyajima Shiplake and wouldn't go back!
The Shilabe (but I like 'Shiplake'
) was demmed in my system in November by the local dealer; on his TD124 deck, not mine, but through my Superline. It was very similar in presentation to my Urushi Vermillion.
I had an Orbe with SME V and after Lyra Clavis and Helikon, took a punt on a used Black Goldline (cheapest K). Loved its full-bodied, musical and involving character. Went to newish Vermillion with its much lower output ( (<3 as opposed to 0.6 mV for the Black and Reds). Lovely ! Still have it but fancy trying a stone bodied K).
Both these cart's worked well in the Five, but the Urushi took off when I changed decks and to a 12" arm. My friend has a 20/3, but with Phantom and Lyras; a big improvement over his former Five and Orbe, it seems.
The Benz Ebony LP is a lovely cart. and was a great counter to my Urushi for some time (I have 2 tonearms); only supplanted by the Transfig. Proteus, which has a similar presentation but extends just about everything whist being a smidgen more neutral.
The SME Five is a very neutral arm; even bordering on the cool, some would say; despite only being at the lighter side of a medium eff. mass (11g), it copes well with a lot of moving coils. The 20/3, by all accounts, is not a colourful deck (house sound, maybe?), and I guess that a colourful cart. like a K, Miyajima, upper Benz and probably others would complement it well.
If you are anywhere near Norwich, or indeed fancy visiting this fine city, I'd be happy to host a session. However, my Dais, all valved amplification and electrostatics wouldn't bear any resemblance to your Naim; this with some surety as I've just moved from 552/Superline etc.
If you can get a used Koetsu with their legendary longevity, you'll be unlikely to lose much on eventual resale. Yes, phono stage etc. is all part of the package, as it were, but one can only concentrate on one aspect at a time, suck it and see.