How track original referrer after external login

I have a site where users can log in with federated identity providers. This means that users can first browse around the site for a while (e.g. domain com), and when they want to log in, they are taken to their identity provider’s (IdP) login page, which can be domain-idp1 org, domain-idp2 org, etc.
After login, the IdP redirects them back to my site domain com/userpage, and naturally this traffic is then shown as referrers coming from domain-idp1 org, domain-idp2 org, etc.

Do any of you have suggestions on the best way to deal with these referrals from IdP domains? I know I can add referral exclusions, but then these visits are counted as Direct instead.

Is there any way to keep the original acquisition source for the visit even after users have been sent back to my site after their login? Does it make sense to use a custom variable or dimension or something?..

This may help Referrer always shows as

Thanks, but the solution for that question just changes the visit from “acquisition - referred by website XYZ” to “acquisition - direct”. So all it does is hide the referring website, which doesn’t really help so much.