The verdict
Pick Brevo. For a lean B2B founder, the deciding factor is that Brevo bills by emails sent with unlimited contacts, so your list can grow without your invoice growing, and it bundles a real CRM with sales pipelines into the same tool. Mailchimp's contact-based pricing punishes exactly the thing B2B teams do, which is accumulate contacts faster than they send, and its CRM is thin by comparison.
Mailchimp earns its place when brand familiarity, its template library, and its sprawling integration marketplace outweigh price, or when your team already lives in it. But absent that, Brevo is the more economical, more complete choice for the way a lean B2B operation actually works.