I know this is a bit of a necro but I just dealt with this myself in this EXACT case. Seasonic PSU cables are too short to reach the upper drives when you have 12-15 drives installed.
My fix for this was to use a 6-pin PCI-E power extension. I bought a cheap sleeved kit on amazon that included a few. One issue is that the PCI-E 6-pin power connector is not the same as the one on the PSU. My guess is they are purposefully different. What I did was de-pin the PSU cable connector, swap that onto the extension, then put the PCI-E 6-pin connector on the psu Cable. The psu cable can now plug into the extension, and the extension can now plug into the PSU. I moved pin's over between connectors one at a time to not get them confused.