Thanks for the comments. I have tried Ente but they want £19.99 a month for only 2TB. It is not enough storage, expensive and they would have access to my files. Mylio looked more promising as you can sync directly with your computer or chosen cloud storage (or both at the same time) but the Android app is full of bugs at the moment.
I already use my Synology but like I said there is no dedicated Desktop app where I can view and sync files with my computer like you can with Apple Photos. You can only access through a browser and it is slow outside of my home network. Viewing on my phone is also too slow and you can barely view the images.
I have tried Nextcloud and may have to use that for now however it does not work well with Live Photos.
It is a difficult situation to figure out. I am surprised no one has done anything about this. If Synology had a dedicated Desktop app where you could download photos to your computer that would be a great solution. The same with PhotoPrism but for now you can only use through a web browser. Mylio seem to be onto a good solution but not working correctly at the moment