The Right Approach to Email Marketing

I came into a very valuable email list consisting of ~60k customers of a management consultancy which is parallel to my company's offerings. A big subset of these contacts could also be interested in my fairly mature software consulting and Excel add-in businesses. What's the right approach to getting in touch with these folks? My fears 1: Getting my domain marked as spam. My fears 2: Turning off the contacts with spammy / irrelevant / salesy info. My fears 3: Needing to diversify my business to add a content arm which tells a story, delivers "valuable" content, and sends 16 emails to every lead via a "nurturing" campaign until 2026. Is there a more effective and direct way which doesn't contribute to today's toxic meta of overflowing everyone's inboxes?