What I find frustrating is that the levels hypothesised to have an effect on reducing the incidence of respiratory illness is way higher than the current very low 'recommended' limit.

Yet when you see studies of Vitamin D they never seem to use the higher level. They seem to stick rigidly to the lower bone health level. Unsurprisingly they see no effect. My own little test[0] showed that inadequacy very easily creeps up on you.

Where is the Randomised Control Test on incidence of respiratory diseases with those people held above the 75-100 nmol/L hypothesis level?

It's deeply frustrating at the lack of actual solid science experiments that goes on in this area. It appears to be a heavy dose of inferred statistics topped off with a heavy layer of credentials, and hope that nobody asks the awkward questions.

[0] https://new-wayland.com/blog/onward-to-100-nmol-update-2/