@Kiicki
Hello my friend, it's been a long time since I sent you a response so I thought I would give it a go. From the sounds of what you and everyone is saying you either aren't acustomed to the high amount of vapor you're getting from the subtank or you're somehow letting your wicks get too dry and it's scortching but not burning the cotton and that would irritate your throat as well...
As the others have said I would trying running your DIY mixes through the Nautilus Mini since you're more used to that and see if they still bother you and make you cough. When I first started vaping I coughed for a couple months as my body was trying to expel all the tar and gunk in my lungs from smoking for 18 years and I still cough dripping once in awhile after a couple high wattage vapes when I feel like cloud chasing and vape at higher wattages than i normally do. It's ok to cough once in a while from vaping but if it's giving you a chronic cough with one tank and not the other then I would stick to the tank that works for you. Vaping is a very personal thing, its about what you like and not what everyone else likes and thinks you should like too.
I know it's exspensive, but if you can't get VG from a farm or feed store like suggested I would definitely get one of those super expensive 100 ml for 7 dollar bottles of VG and see if that makes you cough. It is rare, but you might have gotten a bad bottle of stock from where you got your PG and VG...it's been known to happen from time to time with different vendors, although this would be the first Nic free case of it I've ever heard of I wouldn't rule it out till you have another sorce of PG and VG to compare it to.
Don't give up, sometimes it costs a little money to find the vape that is right for you...I spent hundreds of dollars the first six months, well, actually thousands of dollars the first six months I was vaping because new products were coming out so fast it would make your head spin....and that was when the Protank 1 was a brand new thing and bottom coil tanks were just being thought of and RDA's weren't really common and most of them were a horrible pain in the ass to rebuild and it's a wonder we ever stuck with it and got out of the vaping "dark ages" and "middle ages" where I started but we did and we now have all of this wonderful vape gear that is light years ahead and better than anything we had back then...I simply refuse to believe that there isn't a vape out there that will not be anything but perfect for you. Just relax a little and know that sometimes these things happen, I know it sucks, but with a little patience we can get this situation resolved and you back to vaping happy soon. Stressing about it just makes it worse and doesn't do anything to help but just the opposite.
I hope this post helps and puts you at ease a little about your worries my friend...I know it sucks but get a couple little bottles and mix up some new DIY and try the Nautilus again and see how that works...if you stop coughing then get another Nautilus and problem solved
Good luck my friend, good luck