One thing I’d add from my own experience — besides iCloud and Synology’s native apps (like Synology Photos or DS File), it can also help to use certain third-party apps when you need more flexibility. For example, some third-party app stores give you access to backup tools and file management apps that aren’t always available on the App Store.
If you’re comfortable exploring that route, you can check out Scarlet iOS Store, which provides access to apps outside the standard App Store ecosystem. A few of those apps can make it easier to manage files, transfer data, or even schedule backups in ways that work nicely with a NAS.
Of course, iTunes/Finder backups to the NAS remain the most straightforward option for full-device backups, but combining that with third-party tools can give you more control if you want something beyond Apple’s defaults.