Is this solution, solution approach also sustainable? I see that there were problems with login in Matrix already in 2019 - s.  Can't login to Matrix chat (solved). I am in Fediverse as a user since 2016 - see Hubzilla . Hubzilla is implemented as a decentralized network. An excerpt from the Hubzilla help:
[start of quote=“Hilfe: About (realtime.fyi)”]
Access Control Lists work for all channels in the grid due to our unique single sign-on technology. Most internal links provide an identity token which can be verified on other Hubzilla sites and used to control access to private resources. You login once to your home hub. After that, authentication to all Hubzilla resources is “magic”
[/end of quote]
As a Hubzilla beginner “Single Sign-on made in Hubzilla”  this feels unusual to get used to, but it’s actually like this - I register an account (strictly speaking an identity login-X@hub-Y.tld on ONE Hubzilla node(Hub Y)) and with the identity login-X@hub-Y.tld I can sign-on myself on all participating Hubs - without registration. Provided another hub Z allows me to log in. Which permissions I have after logging in on hub Z is determined by the provider of hub Z.
This “single sign-on made in Hubzilla” is one of the special features of Hubzilla and works flawlessly all the time. That’s why I describe this in such detail as a suggestion. Maybe it is worth to consider in the medium or longer term to apply the solution based on the magic “Single Sign-on made in Hubzilla” functionality also for the “Single Sign-on @ opendesktop.org-services”?
“Single Sign-on made in Hubzilla” is a mature, low-maintenance solution that has been in use for years and does not cause any additional effort for hub admins. Not unimportant - “Single sign-on made in Hubzilla” also feels very pleasant for a user, provides positive user experience.
 Can’t login to Matrix chat, 08.04.2019 - Can't login to Matrix chat (solved)
 Hubzilla - https://hubzilla.org
 Single Sign-on in Hubzilla-Grid -https://realtime.fyi/help/de/about/about#Glossary