I also have an application pending, to replace a Comodo certificate that just expired. It’s different in that I’ve since formed an LLC, whereas the old one was in my own name.
In my case I’m getting an “opinion” letter from my attorney, which I’m told should satisfy the requirements. It was also mentioned that a notary could handle it–but as I read their “template” letter it looked to me more like lawyer country.
I looked into D&B registration, but offhand it looked like they basically want the same stuff that Sectigo does.
This doesn’t answer your “alternative to Sectigo” question; I’m only suggesting you’re not dead yet.