> FCC allows suppliers to self-declare conformance with EMI/EMC rules

Yes, though they can also ask for your lab results or to inspect your testing facility and hit you with massive fines if it turns out you falsified your compliance documents. As you mentioned this is very rare though - even enforcement actions against non-certified intentional radiators are uncommon.

Huh. I thought that is actually the EU that allows self declared conformance, rather than the FCC. But looking into this it seems both do?

I recall trying to build some hardware in 2020 and getting discouraged because of the CE/FCC silliness. Even though I could self declare for the EU it seemed like a big risk to do so.