Your skin releases around 10,000 IU if you spend an hour outside in full sun in the summer. So that, at least, should be quite safe. This article from the Mayo Clinic argues that vitamin D toxicity has been vastly overstated. [1]

[1] https://www.mayoclinicproceedings.org/article/S0025-6196%281...

I was taking 3000 IU of D3 for years. When COVID hit, I upped that to 5000 and have kept it there since.

Depends on the definition of reasonable. D3 is fat soluble and if you take too much for years it can accumulate too much in your body.

I'd say 2,500 IU is max to take daily without infrequent blood tests to calibrate the optimum.

It costs money and won't necessarily do anything about the levels / the things that were causing the levels to be low in the first place. Highly recommend this deep dive: https://soundcloud.com/user-344313169/vitamin-d-mixdown-1

Besides unneccessary expenses? I don't think so. 2000 IE should be fine, and thats with a safety margin of 2.