As for the criticism against vit d supplements because of the extreme variability of actual vit d between brands, regardless of the supposed IU they advertise (there is very little objective, independent quality control being done, which makes it a bit of a wild west, but I guess that goes for _all_ supplements).
I don't think it's necessarily a problem. Ignore the prescribed dosage by the brand. This sounds like bad advice, but it's not reliable cfr. the variability. Just get your bloodwork done first. Then start experimenting with consistent dosages and stick to one brand. Get your bloodwork checked again after a month and proceed from there. Aim at a value between 40 and 70 (according to Gominak, which I concur doing my own research/experimenting).
Disclaimer: I am not a doctor, please do your own research and preferably consult with a medical advisor