I always find it surprising that whenever the subject of backup comes up here, there're always only a one or two posts that suggest tapes only to be ignored. In the present day, tapes are still the best archival backup method readily available. Sure, the entry cost is rather high, but I think it would be worth it in your case since your data pool is only getting larger.
Keeping backup copies on HDDs are convenient, but I would keep a minimum of 4 copies, at least a pair in 2 different sites since HDDs always have a chance of failure when powered. Cloud is only an option if you have no issues with bandwidth. Even without bandwidth issue, I still don't feel completely comfortable subjecting my data to risks entirely out of my control.
Here's what I do personally. All my archival data is backed up on LTO Worm tapes, and I have most of those data on RAID-z2 for availability. Every online data is backed up incrementally daily on a separate array, and I do weekly full backup on tapes on monthly rotation along with month full backup on yearly rotation. Everything that needs permanent storage goes to WORM. While I won't claim my backup strategy is the best for everyone, but I go to bed each night satisfied with the level of safety with my data personally.