Today I learned that there actually IS a way to manually configure an account in Outlook 2013 and newer for ActiveSync instead of relying on Autodiscover, which Zentyal/SOGo no longer provides.
The trick is this: You cannot add the account from within Outlook itself. Doing so will use Outlook's "new" configuration screen that doesn't allow manual configuration for Exchange protocols. The account must be added from the legacy config screen, available in the Mail control panel or by running OLCFG.EXE from Office's program directory (e.g. C:\Program Files\Microsoft Office\root\Office16). After that it's pretty straightforward.
I tested this earlier today using Outlook 2019 and Zentyal 6.2, and it provides mail/contact/calendar sync without the use of a 3rd party plugin.
I have been told ActiveSync has some limitations, and of course the Zentyal admin advises not to do ActiveSync on mailboxes that are larger than 1GB, but does anyone know specifics?