If you can't go beyond 8W with any liquid in a subtank Mini, there's something wrong. It may very well be that you would prefer another tank anyway, but clearly that's not working right. Subtanks don't necessarily do great with 100%/Max VG but that shouldn't be unvapeable. Still, I could see another tank fitting better with your style if your juice is quite thick. Otherwise, even 80/20 VG/PG should be able to go up pretty easily to 25W+ with the .5 head, and easily 20W+ with the 1.2s or 1.5s. Honestly those numbers are conservative, my point is that currently you're only able to use about 1/2-1/3 of the power you could be.
If your juice is very thick, try and find something thinner just to see if this is the problem. You don't have to stick with it if you like really high VG percentages and simply want a tank to accommodate your preference, but it's worth seeing if it helps fix the issue. Otherwise, if this is happening coil after col, it's likely an issue with the tank itself or the Kbox is putting out more power than it's reading. If either or both are under warranty, it might be worth calling the store. You won't be able to respond to this but I would be curious to know if the Kbox is working properly with other heating elements; try it if you have them. If you know how to rebuild, it might be worth trying out the RBA section as well, just to see if it's a stock coil problem.
Otherwise, there are lots of tanks you can use with the Kbox. Pretty much anything that has coil options over .4Ω. Some popular options are the Aspire Atlantis 2, Uwell Crown, Aspire Triton, Joyetech's Delta II, Eleaf Melo, Innokin i-Sub G, and the Sense Herakles. Eleaf also does the iJust 2 tank if you want something inexpensive. All of those tanks have pros and cons but lots of people have them. The Triton is one of the newest and has a number of options you will be able to use with a properly working Subox. Which of those tanks is best is pretty subjective, but I'll hone it in for the sake of making a real suggestion.
Really do make sure you try and pinpoint the problem with your current set-up, because something isn't quite right. The subtank doesn't come with a coil that maxes out at 8W, and I've used max VG stuff in there with OK results (other tanks do better, though). If you just want a new tank, I'd probably recommend the Triton for its brand-spankin-newness and the fact that you can use many different coils in it, or the i-Sub G for its good price-to-performance ratio. Ultimately you will probably want to watch a few videos or read some
reviews to help you decide what you really want out of your tank. Even if you get a new one, though, it will be worth trying to figure out what the problem is with your Subtank so you can have a decent back-up. I have a bunch of tanks and still use my Subtanks quite a bit.
Good luck.