That's actually a pretty nice rig you have there. That burnt taste may be caused by one or both of two things: a wicking problem or too much wattage for the coil.
The OCC coils in the Subtank Mini wick pretty well, but if you're driving the installed 0.5 ohm coil at the max of 50 watts, then without wide open airflow and longer than about a 4-5 second draw, the risk of the coil head not being able to take up enough liquid goes up. Boom-dry hit about halfway through your puff. I rarely push my 0.5 coils at above 40 watts, even on a mod that is capable of 75 watts. The coil just can't keep up.
Try obtaining a new 0.50 ohm coil, prime it per instructions. Drop the wattage down to 17-22 watts and, after taking a few unpowered draws, power the mod and take a good drag. There will be very little vapor at first. Slowly increase the wattage, about 5 watts at a time and alternate unpowered with powered draws. At somewhere between 30 and 40 watts, vapor should be fuller and more flavorful.
Higher VG liquids will take a little longer to hit the sweet spot. After about a half tank, the coil should be broken in.
Higher PG liquids won't take as long--they wick better and can be driven a little harder, maybe even at the full 50 watts your mod is capable of. But as soon as you taste something off/burning, back off the wattage or you'll scorch--and ruin--the cotton in the coil head.
tl;dnr: back off of the wattage and increase airflow while taking shorter drags.