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There isn’t a built-in way to do it with the Dashboard UI. You should be able to disable all other factors via the management API, which would automatically enroll users whose email is verified in email MFA as the only factor.
I haven’t tested this, and it isn’t something we officially support, so this advice should be taken with a grain of salt.
According to our blog post about the initial email MFA release, this is something that we may support in the future. If you want to nudge things along, you can create a feature request for it here.