Almost accidentally I found a workaround to this problem. In an article (from Apple News, of all places) on uses for old iPads, there it was, an app called "Synced Photo Frame by Re-Frame". Free download, but ten bucks once started. It works very much like the "old" Apple slideshow function, able to select one or more of your iCloud albums, set duration, etc. I hate that Apple downgraded their own app and won't listen to its users, but this was definitely worth the ten buck to me. We now have pictures of vacations, family weddings, our grandchild, etc, streaming to a dedicated iPad every day.
No, I do not have any association with this app or the company that sells it.