[seba_dos1]

~5000 IU daily between February and May was barely enough to raise 25(OH) D level in my blood from 9 to 30 ng/ml.

Depending on who you ask, 30 is either the bound between "deficient" and "insufficient", or between "insufficient" and "sufficient". Regardless of who you ask, there's plenty of headroom until "excess".

[rcxdude]

Yes, it's possible but relatively hard to overdose on vitamin D, and due to a cock-up in some calculations in a study, until recently the recommended supplement amounts were about an order of magnitude too low: https://pmc.ncbi.nlm.nih.gov/articles/PMC5541280/