What do you think about the thresholds themselves? How often are they updated usually with new research?

Based on what I've seen while researching the topic for my own health purposes: the thresholds differ in various sources, but in general research appears to suggest that they may tend to err on the too low side rather than too high (there's quite a lot of headroom between what's often considered excessive and what's actually known to be toxic, while the lower end is much fuzzier).

I'm an engineer, not a clinical biologist or researcher. I have no idea.

On a personal level: I used to struggle with "winter dip". Taking Vitamin D supplements, as well as moving to South Africa, has both improved it a lot