Quicksilverboy -
Sharing the same frustrations, I did the same of downgrading back to Mojave. Luckily, I have a backup of the Mojave-based photo library (and iTunes library) before upgrading to Catalina, so I did not have to painstakingly rename/reorganize photos as you did. What a bummer.
However, I chose to downgrade to Mojave 10.4.3 only, as I have read this is the only reliable version before syncing problems started within Mojave (I can’t confirm this as I hesitate to update to 10.4.6 just to see if there is any difference...)
Meanwhile, I can confirm with you that on Mojave 10.4.3, the syncing problems plaguing Catalina, namely inability to sync albums nested in folders or incomplete syncs, DO NOT occur. However, with Mojave, there are other problems depending on what iOS version you are syncing with:
With devices on iOS 12, i noticed that on each new sync of introducing the slightest of changes (e.g. adding one photo in an album), iTunes will remove a small number of previously synced photos, then only to re-add them, which is fine as this is only a time issue where the complete & accurate number of photos are synced in the end.
WIth devices on IOS 13, the same problems exists (removal & re-add), but iTunes seems to remove a larger chunk of previously synced photos, only to be re-added (which is fine again, only a time issue), BUT the worst being that DUPLICATES are now created under iOS 13, inflating the photos that you intend to sync by nearly 50%. Adding insult to injury and unrelated to this thread, Album Artwork of playlists synced on iOS 13 device are also messed up. Clearly an iOS 13 issue when no such problems on iOS 12.
These observations clearly confirm that we are facing multiple issues with the Apple software ecosystem. We have iOS 13 photo syncing RECIPIENT problems with Mojave already, then we also have Catalina photo syncing SENDER problems regardless the iOS version of your device.
Wonder which problem Apple will address first, if at all.