How to track bounces from Sendgrid in CiviCRM 5.x?
OK, found what appears to be the solution:  Airmail from AGH Strategies for "Unified CiviCRM bounce event handler for SMTP services", including Sendgrid.

It wasn't in the CiviCRM extensions directory but it's on GitHub and pretty up-to-date, with simple instructions for Sendgrid.

Will update this answer with our experiences.