At my previous company, we created a "Marketing Qualified" field as checkbox in Marketo and mapped that field to SF. We would have all leads from Marketo go through auto assignment rule in SF. In the SF auto assignment rule, I would put a flow step that assigns them to a rep only if the marketing qualified field is checked. If the lead is not MQL it falls to the very last step in the assignment rule and assigned it to a lead owner queue we called "non-MQL" that way not all leads are assigned. You can set up a flow in marketo saying if they meet a certain score, check marketing qualified and sync leads to SF using auto assignment.
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