For now, I've decided on pCloud, in addition to an on-site copy on my NAS and a copy on a portable drive that I store at work and update semi-regularly. A couple of rsync scripts take care of everything.
I know it's a cloud storage service and not a an actual proper backup service, but they offer 15 days of rewind as standard, and you can get a full year of rewind as an add-on, which I am considering. That should hopefully protect me from accidental deletes, and give me enough time to restore if my house burns down.
The thing that has really sold me on pCloud is that their Linux client is absolutely amazing. Compared to the barely functional Dropbox client and the non-existent Google Drive client[¤], it is an absolute joy to use. At the moment it's an Ubuntu-only AppImage, but they're working on an improved Electron version.
One additional nice thing is that pCloud is a Swiss company, so their privacy laws (and the GDPR) apply. They do host their servers in the US, so you're not completely free from theoretical NSA/PRISM snooping, but in my case I'm primarily storing my music library. They can go ahead and snoop through the tags of 300+GB of music for The Anarchist's Cookbook or whatever.
[¤]InSync is pretty nice, and I did buy a license for it a while ago, but it's still not as good as pCloud's client.