If you haven't already done so, I'd recommend trying the Coimbra Protocol [1] before chasing after brand new research. It has been a game changer for those I know that tried it.
Simply supplementing with D3 would run into problems without that. Nevertheless, they do cite studies showing improvements with lower doses.
Normally I'm pretty sceptical about 'named doctor' websites as they are usually some kind of grift, but this one doesn't seem to be.