if you bought the coolfire as a kit which included the tank then there is no way they will take it back. ive got one isub tanks but never even used it as they are not the best tanks in the world.
all subtanks should be fine using 70/30 mix.
80/20 does struggle in some tanks as its thicker vg and you could get dry hits and burn the coil easier as it struggles to soak up the juice.
tbh i think it might be worth thinking about switching to a mouth to lung tank for the first month to ween you off the cigs using higher nicotine then in a few weeks to a month go back to the isub.
the nautilus is a great tank for mouth to lung which you can use much higher nicotine and 50/50 juice is best as well
https://www.dnavapes.com/collections/tanks-and-clearomisers/products/aspire-nautilus
maximum 14watts with these coils so battery will last a lot longer and so will the juice.
i gave up with this tank after a 20-30 a day habit for 35 years so it sure worked for me.
a month later i bought a subtank and went straight from 18mg to 3mg
average subbing for me is 25-35ml a day.
EDIT:
90vg/10pg thats defo best for a dripper and will struggle in most tanks.