Although an older thread, I thought to go ahead and add the experience I had yesterday signing-up for ID.me, as things went very swiftly! (I began the process at 15.00ish BST.)
For primary and secondary sources*, I was required to submit photos (uploaded by text link from my phone) of the following: US and UK passports' photo pages, a Council Tax bill dated no more than 90 days ago, my Social Security card, and a 'selfie', also directly from my phone. Note: I did have the choice to upload from a different device if wanted.
They were then submitted for review, with a positive reply within three minutes. Then prompted to wait for the video call, with a wait time listed as less than eight minutes. I think this took just four minutes or so before being invited to the video call. Fielded a few very general questions, then had to show each document, some both front and back.
So it seems they've fine-tuned some things (particularly with regard to time needed for the process), which is wonderful news for those open to a bit of a privacy jump to have immediate access to SSA and the IRS. Indeed, within minutes of saying goodbye on the video call, I was able to sign-in to both sites.
Many thanks to everyone who participated in this thread. I'm really grateful to have this new access.
* Edited to add: there's quite an array of combinations of primary and secondary sources available to use (i.e., there are many combinations of sources that one can use to satisfy the requirements, not just the one I've outlined above).