Yes, HDD's should probably be considered medium-term storage. Tapes seem a little more robust, but it seems like M Disc, estimated around 1,000 years, take the crown. Unfortunately 100GB is the limit, so very large files will be difficult.

Not all drives are helium-filled, and theoretically helium just reduces power usage.

Your lubricant drying out will be the biggest threat in my eye as your startup after 10 years may cause a lot of damage on the mechanical bits.