You need to learn about the different ways a backup program can behave, and how to make it work the way you want to. The most common methods are non-mirroring backup, mirror backup and file sync. Arguably, sync is not a backup,
Non-mirror backup simply finds any new files on the data master, and copies them to the data backup. If files are deleted on the data master, no changes are made to the backup; the deleted file remains present on the backup.
Mirror backup duplicates any changes made to the data master on the data backup; new files on the master will be added to the backup, and files deleted on the master will be deleted from the backup. If you delete files only from the backup, and run the mirror backup again, the files you deleted will be copied to the backup (since they are still present on the data master).
Then there is version management backup, which is like a mirror backup, with the addition of maintaining a number of previous versions of a file on the backup device (e.g. Windows File History).
File sync is intended to make sure that files on peer devices are the same at each end; this can be useful in collaborative environments, or if you want to make changes from more than one machine, but have those changes echoed over to the other machine. Strictly, file sync is not a backup method.
You need to decide where the data master is, and where the data backup is. Uusually, the data master is the device on which files are generated, so normally, it will be your PC. However, you may run utilities that save files direct to the MyCloud (e.g. this like Transmission or get_iplayer, that download media).
If you use your MyCloud as both a backup device and a master device, you will need to decide how to perform the appropriate backup, and where to back it up to.
I have personal files (designs, documents, etc) on my main desktop PC. These are also synchronised on the MyCloud, so I can change files using any of my devices, and have them sync back to the Desktop.
I have music media on my MyCloud, that I can access via many different devices. These are mirrored to a USB HDD.
I have video media on my MyCloud, transferred from a Humax, or downloaded from get_iPlayer. I do not back these up.
I have photo media on my MyCloud, transferred from digital camera (by memory card), or uploaded automatically by the MyCloud Android Mobile app. These are mirrored to a USB HDD.
I use FreeFileSync to perform my backup and sync operations, using one control script; FFS can be configured in a very granular manner, selecting directories, and the backup method to use, and where the backup/sync should go.
Since your iPhone and iPad photos are automatically loaded to the MyCloud, I would suggest that is your data master for those photos; use your PC as the backup device. For all other photos, those that are loaded onto the PC, or any other files where the PC is used to create them, or import them or download them, define the PC as the data master, and the MyCloud as data backup. Then find a backup tool, and set it up appropriately. It does require some thought and care to set up, and you need to remember what is master data and what is backup (directory names can help here…), but, once you have a system worked out, it will make your life easier, and your data more secure.