I previously replied to an older post (ipad-os-rdp-smart-card-using-function) requesting iPadOS RD Client support for smart card credential pass-through. The original requested never replied to answ...
Confirmed the same thing. Adding one more device to your list.
I confirmed authenticating into RD Client works but pass-through does not work with:
Identiv SCR3500C https://www.amazon.com/dp/B07CV4D13G
with MoKo USB C Female to Lightning Male (OTG) Adapter connector: https://www.amazon.com/dp/B0CPHVTXQ9