Tailscale now supports custom OIDC providers. But if you already have the ability to host one, you won’t benefit from what’s in the above article (which is about hosting stuff at home even without a public IP address). https://tailscale.com/blog/custom-oidc/

Perhaps everyone knows this, but it's possible to create Gmail addresses without a SIM card inserted. This can be useful, e.g., when one has exceeded the max number of addresses that can be assigned to a single mobile number.

Make no mistake, I am not endorsing third party authentication when making direct connections between computers.

Do you mean creating the GMail address on a phone? Trying create one on a computer seems to require a mobile phone number.

Yes. Remove SIM card. Use Wifi. Create Account. Choose "Google".

Once the "account" is created, can delete it on the phone, but the Gmail address survives.

The usefulness of this is, e.g., avoiding a cap on how many addresses one can create with a mobile number (SIM card), not some increased level of privacy for the user of the phone. It might be an old/throwaway phone. One might never use it for anything except creating Gmail addresses.

The practice of so-called "tech" companies trying to force people to hand over mobile numbers for anything and everything is absurd. It should be prohibiited by law. Mobile numbers are for making calls.

Phone/tablet. You still provide the G. with IMEI of the device, but I assume you can just go buy a second-hand/refurb one for $25-$50 with the cash probably anywhere in the world and toss it later. Hell, you can buy a new one for $50.

But of course the G. would try to get a phone number from you with all the dirty tricks you can imagine. So better use it just as a bootstrap for acquiring some other form of the presence on the net.