Was pretty straightforward converting the freehub. However after finishing I realized the Shimano cassette I removed had what appears to be a spacer (silver ring in pic):
Do you have a XD driver or XDR driver? XD is equivalent to 9/10 speed HG, and XDR is 11 speed HG. That spacer is just a 1.8mm spacer.
Edit: you said 12 speed road, which is XDR. So using a 12 speed cassette would mean no spacer. Using 11 speed cassettes, would mean a spacer, if the cassette is designed for XD. I believe the Chinese made XD road cassettes are XD spacing.
XD would be used for Eagle, and anything before SRAM AXS road. XDR came with SRAM 12 speed road groups, and is a bit longer. It is backwards compatible.
Your Shimano cassette is the 11-34 cassette, which is made to mount onto 10 speed hubs. That’s why it has a spacer. The 34T cassette flares backwards, like the MTB cassettes. It can basically be called “MTB 11” which is annoyingly confusing.
I switched my 11 speed Shimano 11-34 for an 11-28 on my road bike this weekend and was totally confused by that spacer. Wish you guys had started this thread on Saturday . . .