Upgrading DAS units without breaking the bank.

I have several DAS units that are in need of some upgrades. Unfortunately I only have options for RAID 0, 1E, and 5, or I guess I could do a JBOD, but would prefer not to. I was figuring I would just upgrade a few drives to get the most space, but trying to buy matching sizes is proving more expensive. Here is my breakdown:

Drive one - 8 bay all 18TB Thunderbolt 3

Drive two - 8 bay all 18TB Thunderbolt 3

Drive three - 8 bay all 4TB Thunderbolt 3

Drive four - 4 bay two 12TB (had two bays with removable units for portable drives) Thunderbolt 3

Drive five - 4 bay all 12TB Thunderbolt 3

I also have two more empty 8 bay units that I could probably fill with more 4TB drives, but both units are Thunderbolt 2 and a not sure if fixable 4 bay that is Thunderbolt 3.

My current spare drive selection is:

two 24TB

two 18TB

some 4TB, not sure on quantity, probably ten.

I had originally just figured I would buy two more 12TB drives to fill out an 8 bay unit and then two more 24TB drive to fill out a 4 bay unit. Then just use the 4TB drives to fill everything that I have left. The trouble is that 12TB drives are ridiculously priced, so I could buy 20TB for almost the same price (shucking an external unit). Since I have raid limitations, I end up losing space if the drives aren't matching and for all intents and purposes that just increases my price per TB. I need a diagram...