Got it, thanks for confirming all that, you’ve done all the right checks. Since you’re the owner, using the browser version, and the workspace isn’t tied to Teams or a Microsoft 365 group, it sounds like this is a feature limitation on Microsoft’s side. The “remove member” option is still in gradual rollout, so some users can view members and change roles, but can’t revoke access yet.
In the meantime, the only reliable way to remove someone is to duplicate the workspace (which keeps all your pages and components) and then share the new one only with the people you want to keep.
It’s a bit of a workaround, but it’s the current method until full access controls are released.