Selfhosted

Syncthing and nextcloud are not a good backup solution. Like ever. Potentially they aren't even a backup solution at all. Or even cause data loss.

You sadly didn't tell us too much about what you are actually trying to backup and how your infrastructure looks like.

If I understand you correctly you want to centralise the files that are currently hosted on a diverse set of devices into a central file storage on your server and backup from there. Right? That's a fair goal and something I absolutely do myself - and both NextCloud as well as syncthing will help you make the files accessible for devices.

Now,back to the backup part.

You want basically three things from backup: They need to reliable (doesn't help when you can't access your files anymore because they are corrupted), you want them to be as unaffected by any potential risks as possible and let's face it,you probably want them cheap. The second part basically dictates that for an online backup you want something that can do versioning so corrupted data (e.g. from ransomware) is not simply written over.

My current approach is: I have an internal backup server (see below), an external backup in the cloud, and a cold storage backup in a bank safe. Sounds like a lot? We will see.

Let's look at cloud storage first. There are a multitude of solutions available for free with Duplicati, urBackup or goMFT being some fairly popular ones - I personally use Duplicati. These periodically scan the folders for changes, encrypt the files and send them to a cloud provider of your choice (e.g. an S3 bucket.) and to some extent can also do the versioning. (Although it's safer to regulate that via a bucket policy as otherwise the application needs delete rights - which means in theory could delete all the data when compromised). Main benefit is the ease of access - you need to restore a single file? Done fast and easy. Not so much for a whole setup, restoring things can get quite expensive.

If you use ZFS there is also the option to use ZFS sent to backup, but as there is currently no reliable European Union ZFS sent provider I am aware of (rsync.net does this,but is US based) legally cannot use them. So no experience on that.

To backup clients completly and VMs/LXC it might also make sense to use a designated backup server,e.g. the proxmox backup server. These do require local (as in "where the PBS is running" storage, though, so a local PBS and a cloud storage behind doesn't work. (There is a "hosted PBS" Service available, though from Tuxis. They work really well). But it can make sense to let a zimablade run a few old hard drives for a few hours a day for that.

For offsite and online backup - as a full restore is always expensive and time consuming from the cloud- I also use two USB hard drives. One is always stored in a locker in a bank vault and every few months I change drive - so in case of a full server loss I only would need to restore the state of a (at max) 4 month old server via USB and then update stuff from the cloud for the 4 months after that.

Now, to be extra sure I also burn the most important files (documents about the house,insurances,degrees,financial and tax data, healthcare records, photos of lifetime events, e.g. weddings, birthdays,births, graduations as well as "emergency data restore howtos", password files, basically all the stuff I want to make sure my heirs/kids have access to if I die) on blue archive (important, not normal disks!) M-Discs. They are supposed to last far longer than normal blue rays and most consumer accessible media. These are stored locally,in the safe and at the court that holds our will. The reasons for that? Powered off hard drives lose data quite fast and if the wife and I perish at the same time, eg. because we have a car crash or the house burns down the issue is time: Cloud backup might not be available anymore as our bank accounts are frozen and therefore the backup is no longer paid for. The bank safe is not accessible for a long time for the same reason. When someone then accesses the USV drive it might be of no use. The server might be powered off or damaged. And sadly the legal system here can take years (up to 7 years are my planning times) before they can actually access the data.