I read your spreadsheet numbers with great interest. I had to Google what a 2010 Ford F-150 FX4 5.4L FFV was as big V8s engines like this are something of a rarity over here. Am I right in thinking this MPI, not GDI and there's no turbo?
To my eyes, you have a perfect set of UOAs. No oil thickening, no wear metals and naff all fuel dilution. Yes some of the TBN numbers are low but that's about par for the course. TBN always depletes quickly and it's not something I'd necessarily worry about if KV100 and Iron are okay.
Regarding the 8.9 TAN, the first thing I'd do if this was my test would be get a recheck. People think TAN & TBN measurements are so routine as to be infallible but that's not my experience.
If it did check out at 8.9, then I might think to myself that The Big Engineer In The Sky is tapping me on the shoulder and telling me to watch out!
When oils 'break', they do so astonishingly quickly. I've see oils close to their end-of-life go from apparently normal to 'too viscous to measure' in the space of 20 hours.
If you did run the next OCI out to 20,000 miles, and got another 8.9 or slightly above, I wouldn't necessarily conclude that the TAN had stabilised. Sometimes TAN just stops rising (even though logic says it should) and it usually means that the oil is actively dumping sludge. I've no idea why oil behaves like this; I just know that it does.
One final thing. I looked at a YouTube clip of someone dismantling one of these engines and putting in new pistons and rings. The rings look seriously thick; just what you want for good cylinder sealing. I think what this says is that if you don't play fast and loose with the mechanical bits, then these low viscosity oils work just fine.