Yup - I know, I need to replace ALL drives before I get the new size.
I might be over thinking it -
I'm hoping to buy drive spread out because then they won't fail all at the same time vs buying them in one shot (which I did initially)
In order for that to happen you'll need massively bad luck, more so related to the particular batch you received. For example if they were magically of lesser quality (not likely imo) or poorly handled in the supply chain (more likely imo).
Buying at different times (somewhat in the same ball park) from the same place does not guarantee you're free from the problem.
I remember facing the same "optimization craving" as you, several years back. I bought from different vendors, and spread out the purchases.
I wasnt as impressed when I found out the manufacturing dates ...did not match the same timeline as I procured the drives.
Ie, the amount of control I hoped to gain, was lost in inventories out of my control.
I will never bother with spreading out in time again, but I'd consider buying from different vendors - only to spread my chanes in terms of supply chain handling, and letting inventory-turnaround times be the "time factor".
Good luck.