I certainly wouldn't recommend to use your primary domain name for sending out large e-mail blasts that might get marked as spam (whether they are "legit" or no

I certainly wouldn't recommend to use your primary domain name for sending out large e-mail blasts that might get marked as spam (whether they are "legit" or not). An alternative might be to set up a subdomain (e.g. marketing.example.com) for those outgoing e-mails, create your SPF RR for it so that only MailChimp's sending IP addresses are permitted, and let your users use their preferred tool (Mailchimp).