Thanks for doing the test.

The problem is actually the lockring !

As explained, Moulton used a similar thick to put a 10 teeth cog on a proprietary modified Campagnolo freewheel but could keep the standard Campagnolo lockring and the standard Campagnolo tool. Maybe because the smallest cog is bigger, 10 teeth vs. 9 teeth ?

A custom big washer between the Sunrace lockring and cassette could maybe do the job ?