I've always been wary of going up to max-dose levels for that reason.
It's a good idea to get your vitamin D status tested yearly and aim to be at least 50 ng/ml (120 nmol/l), but not more than 80 ng/ml (200 nmol/l).
As an example for dosages: I take about 7000 IU of vitamin D3 and 180 mcg of vitamin K2 daily. My levels test at about 180 nmol/l and my blood calcium is normal.
as mentioned though, i only think that 500-1000iu is not enough for most people. I'm not a doctor though, but it is one avenue of alleviating SAD as well as lethargy and stuff that most people vape/smoke/caffeinate to deal with.