A network bridge (in this case) is a virtual interface that joins one ore more network interfaces. It can be compared with a switch. If you add an interface, in this example eno1 (your physical network interface) to the bridge, it is called a slave of the bridge.
The idea is, to add the physical interface to the bridge (this is, what "nmcli con add type ...
As far as I can tell, this option max_parallel_downloads has not actually been implemented. I grepped through the installed Python source on my Fedora 30 laptop, and then searched the project's github repo, and found only the references in the documentation.
I would recommend filing a bug report.
389 documentation on the basics like upgrading is rather sparse, as they defer to Red Hat Directory Server. Which on a different version number. Even with all the confusion, the Red Hat DS docs regarding upgrading from the early versions (7?) may be useful.
Regarding schemas, pre-migration tasks includes copying over the schema files.
If you had an ...