This is the most United we have ever been historically speaking.

I think this depends on your timescale. If you’re looking at the past thirty or so years, as far as I can tell political division is on the rise. If you’re looking at the past 150 years to include the Civil War, then of course there’s a downtrend.

There are no legitimate independence movements

That depends on your definition of legitimate. If you mean legal, then of course not, since secession is almost universally considered unconstitutional. But there are definitely sizable independence movements in several states. None, I imagine, would come anywhere near 50% in favor of secession if a statewide vote were held today, but this could certainly change over the course of a lifetime.

We have the worlds most stable currency (we have the world reserve currency)

True, but I don’t think that’s necessarily a good case for the continuing strength of the Union. It means an unstable currency won’t likely be a cause for any states declaring independence, but I doubt a state which considers secession would say “but wait the currency is so stable” and thus decide against.

Political involvement is the lowest its ever been (which as terrible as it sounds is still good for stability... a lot less political violence if no one cares about politics)

We’ll see if this continues. That trend can easily reverse. I think significantly higher turnout is quite likely in 2020.

So... no, there is no real risk of that actually happening. If anything that odds of that happening are decreasing.

I think odds are probably low. There is vanishingly small risk in the immediate future. But I also think you’re far too optimistic. A lot can happen in a lifetime.