The safe “official” upper limit is 10,000iu a day. Notice I said “official”. You would have to take 40,000iu or more every day for several months before you MAY become toxic. So the 15,000iu is safe.
If you end up with any “symptoms” come back and tell me.It usually means a mineral like magnesium is low. Since you have been taking 5000 iu I do not expect an issue.
Reasons that you may not have made it to 50ng/ml on 5000iu would be a weight greater than 150 lbs, not taking Vitamin D with food that contains a fat, you have some kind of “gut” issue, medicine interaction, gallbladder removal, brand of Vitamin D and if you are already battling a disease or condition.