That sucks, I was about to do the same (Jellyfin on the Synology, Apple TV for streaming). Seems there is no alternative to Plex that works with an Apple TV...
Actually, the Infuse app is almost perfect for Jellyfin on Apple TV. Highly recommended if you don't mind the small subscription. It takes load off of the server by playing a wide variety of codecs natively. There's also an OSS app in fast development that looks promising, but only Android/iPadOS for now.
