Hi Leon, Once a UUID is assigned to each array during raid creation it doesn't matter what the /dev/sdx order is if you specify by UUID in mdadm.conf... madadm scans all drives looking for the UUIDs and uses that to ascertain which drive is which
Did you create the script to change the disk time-out and place it, for example, /etc/rc.d/rc.local so it runs when booting? Those drives of yours are not suitable for use in Raid 5/6 without doing that.

No idea how you are setting the drives up - never watch a "How-To" on YouTube or anything else similar - can read far faster than any narrator can speak and thus lean a lot more in the same period of time... and you can always refer to any written part instantly. Lot better than having to replay something to make sure you heard and understood a certain passage correctly... Initially several years ago downloaded and used the Redhat Administration Manuals and went from there... Studied the complete set, beginning to end, while going to//from work on the train.

If you continue to have problems - then it might be time to look at the hardware... would be inclined to ditch the two budget PCI/PCIe controllers and get a decent 8-port one with correct Linux support, eg a modern LSI, assuming one of your 8x or 16x PCIe slots is vacant... are you using good quality SATA cables with clips? The old original ones with no clips are notorious for creating intermittent connections as are cheap controllers that don't have the little notch for the clip to latch onto. Wouldn't be surprised if your two add-on controllers fall into that category. No clips means you are relying on friction - you might get away with it - but the number of drives you have provides more opportunity for vibrations and connector movement...