SSO with custom login

Hi, I need to integrate SAML SSO with multiple custom branded login pages, we are not able to use Universal Login due to some business requirements, and we are having this error when we try to integrate it manually.

For context, not only we will be the service providers, but also other partners as well, so we need custom experiences for those and that’s is not scalable with Universal Login.

Is there any way to do it manually? We’re not able to figure this out.

Note: the other service providers will be in other domains.