The "too many domains" is similar to one of the main motives that brought me to build kaimail.net. The other motives were to allow the people in my open source community to actually use their custom domains for email, so initially we're doing email forwarding and now have SMTP sending.
But as noted mailmark.dev's pricing is a bit high though. The tracking feature looks nice, but SMTP2GO also has that even with their free plan for a limited amount of domains (3 I think).