The Plone multisite with twelve websites of the Austrian Architectural Forum is now running on 5.2.2-pending/py3.7 (coming from 5.1 latest) without any known problems. [*some are lacking the DNS switch)]
This was some effort due to the large amount of add-ons and custom code used. Some were already migrated, others migration was done or at least finished by us.
The sites themselves are
collective.lineage sub sites (with a bunch of lineage add-ons). Mosaic layouts are used without the editor (content and site layouts) and a very custom theme avoiding Diazo as much as possible.
webcouturier dropdownmenu was replaced by
collective.navigation already in 5.1, now Plone native drop-down menus are used.
Lots of custom behaviors, indexers and types with clever used collections had Bytes/String problems and some custom migration steps were needed.
The process to migrate the 3GB DB (RelStorage 2 with an updated PostgreSQL 12) and its 130GB blobs was smooth once we figured out the exact steps.
zest.zodbupdate was very valuable and some custom rules were needed as well.
The whole runs now in a Docker Swarm with Traefik web-server (which I really love for its simplicity and convenience) and Varnish cache. At the moment 6 single threaded instances are running, but still watching the performance monitors if this is sufficient.