Your assets are on LTO-5 tapes and LaCie rugged drives. The LaCie drives will run about 80 MB/sec and the LTO tapes about 100-120 MB/sec for retrieval.
Today, bare SATA drives of 20 TB each can run at two times this speed even over USB3.0. Bare drives run about $ 16 per TB. Many drives in a RAID enclosure, run about $ 25 – $ 40 per TB. This is your cost per TB for 5 years of storage.
Your 200 LTO-5 tapes will become 16 LTO-9 Tapes if you migrate then now. So, restoring the Retrospect LTO-5 tapes to hard drives, and then writing them to LTFS LTO-9 tapes will extend their life.
After 5 years, this cost of hard drives will halve, and most likely, take up half the physical space as well. So, your 300 TB of data, which takes up 15 drives of 20 TB now, will fit in 6 drives of 50 TB each of which will most likely cost exactly what you paid for the 20 TB drives now. If you paid $ 5,000 for 15 drives of 20 TB today, 6 drives of 50 TB will cost about $ 2000, 5 years from now. And they will hold the same amount of data. (I’m basing this on the fact that in 2017-18, 8 TB drives cost nearly what 20 TB drives cost today)
As for your clients, your best bet is to catalog their data using something like DiskCatalogMaker and send them a mail, showing their data as a pdf (made from DiskCatalogMaker), and offering them a certain amount per TB for 5 years. I suspect this will be less than what they would have paid for a cloud service. You could also let them pay annually.
Even if you bill your client $ 10 per TB per year, some clients with about 10 TB, may even be willing to pay for 5 years up front. And even if you manage to convince 100 TB worth of clients, you’re looking at recovering part or all of your costs for this storage venture.
There is, of course, the cost of the labour of doing this retrieval and migration.