I have been looking around and I cannot find anywhere that sells a cassette with a 16 tooth top gear that is Shimano 11 speed compatible. Junior racing requires gear limits and I was hoping to find something similar in spec to Ultegra or Dura-Ace. Please can someone advise?
https://www.velonews.com/2015/10/news/technical-faq-junior-gear-options_388340 - Junior racing requires a maximum rollout, which is often ,incorrectly, interpreted as a 15 or 16 small cog on cassette. A smaller chainring does a better job in most cases. Its probably a 'thing' about size from a sport drenched too much testosterone.
A 34/11 is nearly the same as a 50/16 (I'll let the author do the math for whats legal with his wheel and tire size.) Compact or even MTB cranksets can do this easily. Drop the large chain ring making it 1x (You probably have closer gear spacing than an adult running 52/36 on a typical road cassette), select a chain ring that gives you the maximum rollout you are allowed on the 11, and cassette that meets your low gear requirements. I'm not up in the play with road/drop bars setup and maximum cluster sizes, but the Gravel bikes will almost certain have gear that's got you covered, certainly 11-36 if needed and maybe even 11-40.
It sounds like you want a 11 speed cassette which is Shimano compatible and has smallest cog with 16 teeth.
Miche makes some, and seem to be the only people who do.
However, Leonard Zinn notes that he was able to swap Campy, SRAM and Shimano wheels without problems on 11 speed (so if you had a Campy 11 speed wheel, it would seem to work).