Just in case anyone else lands here with the same issue.
Here’s what I found first that led me to a fix that has finally worked:
https://discourse.roots.io/t/aborted-trying-to-redefine-constant-multisite/17107/4
Which then led me to this post by @knowler:
https://discourse.roots.io/t/not-sure-how-to-get-multisite-functional-in-dev-environment/15701/2