If I understand correctly, The most storage you will ever get is less than 60T and that is with 5x12T drives. Not worth it at the current price.

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Also, I am confused why you would want a Raid enclosure (your 5N) to function as an unprotected external hard drive. You only have 19.97T available as you are protected across your 5 drives from a single drive failure. If you could, switch to an unprotected setup and have the raw capacity of 25.46T available, you would lose everything if a single drive failure. Why would you want this?

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I prefer the Dual Disk Redundancy (allowing for 2 drive failures), which in your case would reduce you to 14.52T available. But it gives me time to order a drive with Amazon Prime and replace the bad/full drive in case a second drive goes as well, and only if a 3rd drive where to fail would I lose any data.

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In my opinion, you have a few options.

1. Buy normal External Drives, with no cloned safety (aka Non-RAID) and move your content to them. You could get a few $140 each ($17.50 per Tb) Seagate Expansion 8TB Desktop External Hard Drive USB 3.0 and plug them into your Router.
2. Buy bigger drives for your 5N. Seagate 8TB IronWolf $240 each, you will need 2 to gain any storage.
3. Delete data you don't need.
4. Buy Cloud Storage (Google, DropBox, Amazon...) and store data you don't need at the moment.

Good luck.