Bringing in outside managers is an anti-labor practice. Especially in the context at hand, where we're talking about a company's first EM.
"What if they ruin our culture?" Promote from within, so you can be sure your first EM exemplifies your culture. If you don't think your own employees exemplify your culture, you don't have culture.
"What if we lose good people because of them?" Promote from within and backfill with a new junior hire. If your engineers are so hostile to each other that they can't stand the thought of one of them getting promoted, then you have a dysfunctional company with a hostile culture. If you yourself have the prejudicial belief that your employees will get jealous, then you're actively supporting the same anti-labor culture that says that salary transparency is bad because it makes employees aware of structural inequity.
"What if they cause harm?" Promote from within, and you can be sure that they have far more historical context and product insight than a random person you might air-drop into your team.
Unwillingness to promote from within says far more about the founder and the labor-hostile culture they're building that it does about effective engineering.