I might suggest, even though you have a big box mod already (which is fine, most mods and tanks are interchangeable these days, looking into a non-smok tank, they can be really difficult for a variety of reasons, including not all coils being yanked when they should be, difficulties priming and etc.
If you wish to stay sub0hm, I'd recommend a Crown Uwell tanks (the original if you can find it as it has the most coil variety, from quite low up to 1.2 resistance which could give you a "taste" of MTL vaping (although airflow and drip tip are set up for sub0m vaping but this tank has some flexibility in airflow options too). It gets great reviews from users and for flavor, generally. The coils tend to last.
If you want to try MTL vaping (higher resistance, less vapor, more cigarette like draw) you could also try a MTL tank on top, you just would be greatly reducing your wattage needs, which is totally fine. Sometimes MTL vaping is a bit easier on the system when starting out, although there are also many vapers who started out sub0hm and like it, so it's really just another option.
Some good MTL tanks that might work:: The nautilus 2, has coils 1.8 and 0.7 it's a small tank, but not a crazy thirsty tank. To avoid leaks, you MUST fill to the fill line. It was my quitting tank.
I sort of have a hunch you might enjoy the Innoken Zenith tank, it's 4 ml, easy topfill and good flavor (comes with 1.6 and 0.8 coils, the 1.6 coil has about a 2 tank break in period, the -0.8 is said to be a flavor monster out of the gate). Coils also last forever, airflow's pretty adjustable, and it's just a good, all around flavor tank. I am still vaping my original coil, I can't get it to die! LOL. But, that is a nice, solid, large MTL tank that would look good sitting on your smok mod.
I wish you luck.. It's really not fun fighting with one's equipment, no, and does tend to want a person to want to start smoking again, so I'd really, genuinely recommend trying a different tank as you are having trouble with the one you have, and I don't want to see you fighting with your equipment forever. . Good luck,
Anna