If that's really what you need, the relevant jurisdiction is likely to have some adopted e-identity system.
If not, go back and question how you can solve your issues with as little PIIs as strictly necessary.
If you don't like "accounts of well-known service providers" -- email or login-with-whoever then sources of identity that "everyone" has that are hard to get many of are government ids, phone numbers and credit cards.
Like what else is there? For super technical people we could do something like "proof of bitcoin address" where we verify control of a wallet with some minimum long-standing balance. We could make you submit a recording of yourself saying some random phrase and then do some facial recognition to detect duplicates. IP banning hasn't worked since the 90's.
Seems to me that 1) Gmail accounts aren't that hard to farm 2) most spam comes from free webmail providers not disposable email and 3) a captcha will solve most of your problems and if it doesn't, it means your site is being specifically targeted by someone with cash to burn, so you can assume that they'll have emails that aren't on the blacklist. At that point, invest in detection and moderation.
This is 100% not true. Yes, the main site will require it, but there are legacy "portals" that allow signup without a phone number. No, I won't link to said portal.
As of gmail -- one of the guidelines for the list addition is whether you need to go through some sort of a registration. If you do then it is usually not treated as disposable by us.