Nitrox certification...

[AfterDark]

Very surprised that you didn't use Nitrox much. But I suppose it also depends on how many repeat days diving you do, and the working depth.
The second time I went to Scapa Flow for a weeks diving I was Nitrox qualified, as was my buddy. Very unusual at the time (early 90's). Nitrox was only available from one skipper only (not ours). We used to nip down to his boat early, get out 3l cylinders filled with Nitrox 50, and use this on the (air) decompression stops each day. This massively reduced our fatigue, especially towards the end of the week.
The following year, we had Nitrox clean Twins, and had Nitrox each day, that massively reduced our decompression penalty.

Now, if we go to Scapa, we make sure everyone is Nitrox qualified. It's safer, more enjoyable, and reduces our decompression penalties.

That said, Scapa is perfect Nitrox diving.
Capital wrecks between 25m and 42m. Block ships are generally shallower than 20m.
It's two dives a day, capital ship in the morning, block ship in the afternoon. Temperate water diving, 5 days back to back. Decompression diving is the norm.
If on twins, we generally put a rich mix in for the afternoon dive (36%), then air top for the following morning to ensure a safe PO2. That keeps the cost down. Nitrox is blended, so cylinders need to be O2 clean.

Because I've been diving a CCR since early 2000's, I only now buy Nitrox for bailout, or when on Holiday and OC diving.

When I went to NC in 2013 I tried to use nitrox but got burned the 1st day by the weather and changing dive sites. The max depth made the mix I had that day sketchy for the secondary sites chosen on the fly when the weather wasn't what was expected. I was doing 2 dives per day and used 21% for the rest of the week. I did do some lite air deco a few times after exceeding NDL; I used doubles there so I could exceed NDL and get away the deco. The DM in the water thought I was hanging taking pictures, which I was, while I decompressed.

Most of my diving is one dive per day many 2-3 days some weeks and rarely past 50FSW. I dive locally 99% of the time and live 1/2 hour from some dive sites.

Getting to and into the water from shore is getting harder every year. After one dive the thought of having to gear up and do it again just seems like too much work.