3 harsh = temp
There are generally three things that can contribute to harshness/throat hit.
1) Higher PG%
2) Higher Nicotine levels
3) Citrus jooses containing higher citric acid in the flavoring.
There are a couple other factors that can exacerbate the issue.
Heat. If your vapor is at too high a temperature for your airflow, the heat from the vape can cause it to burn a bit feeling like an almost dry hit at the hotter temperatures. turning down the watts on a variable device can easily eliminate this variable.
The Less Pure the VG is the harsher the hit is. For example, certian flavors from White Label Juice Co are too harsh for me because of the combination of a less expensive grade VG and higher levels citric acid from fruity flavors (one example for me would be double apple).
Type and density of whicking in rebuildables can contribute too. I'd recommend sticking with organic cotton.
I went through something similar before discovering I was one of the small percentage of the population that has a reaction to PG. The most important thing for you to do is to find a way to eliminate one variable at a time and track the effects on your experience. Right now it sounds like you're all over the place trying to figure out what was going on, just like I was until my OCD took over and and I started breaking it down into all the different variables. One thing you can do is find some super pure VG premium Joose in a non citrus flavor and vape it at medium to low wattage (producing 3.8 to 4.2 volts) and find out if its the VG that's causing your cough.
The real problem is you're introducing a foreign substance into your lungs and they're going to try to reject it. You've conditioned them over a longer period of time to tobacco smoke. Bet you coughed on your 1st cigarette

It may take a long time to get used to vaping, but you can speed up the process by using what I've offered to find the best option for you. Hope this helps.