Hi there,
So to start, I’m new, just recently jumped into NextCloud. I know how Raspberry’s work and have let’s say average knowledge about linux code.
I recently installed NextCloud for the first time on a RPI 3+B.
I’ve aways used my synology for cloud related things like sharing photo’s between phones and computers, but the synology app lacks functionality.
NextCloud looks really nice and for what i’ve seen the app is amazing 
The thing i’m missing/not seeing is:
What i would like to accomplish is that the user data is not on the SD card or any HDD connected to the PI but on a shared folder on my NAS.
So when i create a user it has by default 2 folders (Documents & Photo’s). Is it possible that these 2 are created on that share by default?.
If not.
How do you manage backup’s?. When i connect a HDD with USB, store all data on it, and it fails after a month or 2?
Can you post the name and version of your synology ?
Perhaps you can install Nextcloud direct on it.
I have a Synology DS218+
Ive installed it within docker, but after creating some users i can’t find their data location to backup. I also like to backup it to another device. Just to be sure
But if i run it on a PI4, how can you make a safe backup on just a single usb attached storage device?.
I think it is better to use Synology DS218+ with nextcloud on the system. Solve your problems with it.
But first you must answer the question:
Where is your data and where is the backup?
Alright, but were will the data be stored within docker?.
Ive looked for created folders but can only find the docker container
Sorry. I do not use docker and so i am not a docker expert. Perhaps you can ask in another thread detailed.
But do you have a backup of your Synology or is it the backup for your data?
My synology is in raid, so a full backup of my nas, but id like to also backup just the user data. Got 4TB of movies that are not so important 
You have a synology and a nas or is it one system?
raid is no backup.
no backup on user error, …
It’s one system.
You are right. It’s not user proof.
But. I have a 4TB external HDD. How do i manage a safe backup if i connect that HDD to a raspberry?.
I’d prefer the raspberry because my NAS is used for other purposes
If you want to use your Pi for nextcloud and your synology primary for nas (SMB/CIFS, …) you can use the synology in nextcloud for primary (path / for user in nextcloud-gui) or secondary storage (path /nas for user in nextcloud-gui).
Not tested:
All Data on external storage possible?
If not all data you can define subdir.
Perhaps it does not work.
Please use if possible smb/cifs for access from Pi to synology .
https://docs.nextcloud.com/server/19/admin_manual/configuration_files/external_storage/smb.html
I think i would use Nextcloud direct on synology.
I also think that it is possible to use Nextcloud and NAS both together and separately.