Building a Covid Community response app - need SSO?

I am building an app that some public services (with various users) may use.

Previously I have used Google as SSO on a client app but this was using their Gsuite domain account.

It was pretty simple to setup as outlined here 

I am wondering what the best way is to provide secure/robust access to the knack app might be

1. I could just use the built in access/user mgmt in Knack         OR

2. I could use some SSO solution which has the option to turn on 2FA

Has anybody here done that with freely available resources and if so how

e.g. If you try to set this up with a free gmail account you get the screen below ..waiting 4-6 weeks and going through an independent review off app really not a runner


I sense people will be uncomfortable using twitter or facebook for authentication even though I wont be sharing data with them your average user wont understand that when you ask them to "login" using facebook


Any ideas ?