@regis In OpenCraft deployments we’ve been using Mongo 3.6 for quite a while now (on Ironwood).
This is the PR that introduced the MongoDB 3.6 support:
And it can be cleanly backported to Ironwood.
Also, there’s a discussion about the Mongo upgrade from a while back here: How do openedx operators upgrade Mongo? Do you use the upgrade playbooks in configuration?
Btw, Mongo 3.2 is still supported by the newer pymongo version.