I think that Syncthing and SeedVault seem to be the most accepted (at least I keep seeing them most often) cloud backup solution - both of them only support NextCloud though. I can't find a reliable open-source background backup app (RCX uses rclone, but it has no automatic backup scheudle) that uses other cloud providers (e.g. google drive, backblaze, aws glacier), so I'm tempted to just give up searching and use NextCloud.
I'm concerned though that Nextcloud is written in PHP, which is known for a lot of security vulnerabilities. Setting it up will be a major effort as well (for me at least as I've managed to avoid setting up my own server so far), so if someone could confirm that it's in fact a safe option to use for backups or if it's just used because there are no better alternatives that would be really helpful.
EDIT: link to CVE vulnerabilities list for reference