How should I store my HDDs?

For the case, I see it would be beneficial to consider 3-2-1 rule with RTO and RPO in order to get planned Backup and DR sites.

For a backup and DR boxes, I am using 2x Synology, so I can suggest you to take a look what they got. Veeam Backup software is used to backup/move data to backup NAS and then to DR one (that is in other building).

I found the hardware solution from aclouda that seems to be more beneficial for your caser. I see it allows to use cloud storage as local SCSI drive thus it allows to run backup jobs to cloud as it would be local drive.

Hard disk isn't designed to store data for some reasonably long period of time. For vaulting you need disk -> tape and use something like Iron Mountain and similar services. Companies who need to mess with regulatory requirements do this for years.

Don't store your backups in off-site hard drives. That sounds like a dangerous and inefficient way to store off-site backups.

I'd store your on-site backup on a professional NAS or SAN with disks in RAID (based on whatever you can afford) and then use a cloud service for off-site storage. A cloud provider like Backblaze (no, I'm not affiliated with them in any way) is really quite a cheap way to store an off-site backup when you consider the costs of purchase hard disks, upgrade, and maintenance. You would just upload your off-site backups.

Should you stick with hard drives off-site, store them the way they should be stored, in an off-site server in a RAID configuration and upload to that from your primary site. Moving mechanical disks on a regular basis isn't a good idea.