How can I enable self-serve sign in / check in to an Event?
I had the same process in place before started using civi - zoom integration.
The process looked like this:

Create webform to update participant (an existing contact) with checksum.
Use google qr to generate the QR code with token so that by scanning the qr code takes you to https://your.org/node/2?cid1={contact.contact_id}&amp;{contact.checksum}
They can click submit to mark themselves as attended (or you can even make this form auto submit with simple javascript).
You can set that thank you page after submission is actually a redirect to the online conference link or wherever.

It's a bit hacky, but it works.