How do you measure the 0.6mm gap when all is shimmed out?
With a gap of 0.7 and an endplay of 0.1 your gap will be 0.6-0.7. Probably when you have shimmed it al you will tighten the nut what can result in a tiny bit smaller gap but still within tolerances.
When mounting a new shoe it is OK to use a gap of 0.6, the first run will re-shape the shoe and start some surface wear it will create automaticly a gap of 0.7.
I have found these brass rings in the hardware store between the brass nuts and bolts, so far no issues with it than only they are about 0.5mm smaller. Ceramic you will not find but if you know someone with a lathe you can purchase a piece of real bearing bronze or ceramic and make some rings. For sure you will never have a failing thrust bearing again
When using a ring then do not use the zero endplay setup. The endplay with it is less critical, just be sure the clutchbell is not touching the shoe.