Custom roles is the only thing I can think of (and I didn’t see any other settings that would enable it)… Sorry that it’s blocked for you. Sharing below in case it can be resolved.
You could automate the assignment of a custom role to each user upon creation. Or even just used the bulk action in user overview / segments to (backfill all users, and then...) add it each day for new sign ups.
If the technical blocker is on SSO bring over unique or disparate roles (ex. staff vs customer, etc), you don’t need that level of specificity for this solution. You just need a single custom role that is applied to all users. And the action need not happen upon sign up, it can be delayed a bit and triggered by a webhook or applied manually via the bulk actions for users.