I'd vote for an iDrive or SpiderOak plugin. I think at least one of them has released its framework as open-source, and security shouldn't give way to convenience.
Or public cloud storage + native encryption from OMV (including file's name), in order to get the low price of these services and still retain security. Synology OS has an imperfect version of that (file's names are kept in clear).
I worked on a Spideroak plugin and tested their framework at the beginning. While they fixed the bugs, the performance was terrible (not because of internet speed either). So, I stopped. Backblaze B2 looks promising and is cheaper. I haven't tried it to check performance though.