Hi
@Seb Johnson and welcome to the planet.
years ago smok were ok until they started flooding the market with a new mod or tank almost every week. since then their QC when down the pan and coils for the past couple of years have been hit or miss and some lucky to last a day. last tank i used was tfv4. i did buy the tfv8 and gave it away within a couple of days.
there are also shit tons of fake coils out their so best never worth buying from ebay or amazon unless they are linked to a well know shop.
if your juice is quite sweet and has a lot of sweetner in it then that would be the only other thing that would make the juice go dark especially at the wowza wattage you are using.
i have many coils that are rated 80-120watt but im quite happy using them at 50-60watts and get plenty of flavour and cloud. even at this wattage the coil is black after a day or two.
you mention you prime new coil and start at a lower wattage. what wattage is that you start at. for me its at least half the wattage i will finally vape at. in my old crown 1 & 2 tanks i vape at 50watts but start at 25w. you will get no flavour or cloud but the coil is still heating up inside so slowly burning it in. turn up a watt every couple of puffs so it builds up the coil heat slowly and the coil should last longer. most prob will still see dark juice after the first day though.
when you do decide to upgrade one of the best tanks for flavour out there is the freemax mesh pro which a lot on here use and rate highly.
hopefully others will be along to advise soon.
edit
just reread your post. adding nic shots wont oxodize that quickly but you do need to shake the shite out of the bottle until your arm almost drops off. the more shaking the better and then let settle. can take a couple of hours for the bubbles to settle. some people shake the bottle every time they fill their tanks but i dont bother doing that.
edit of edit
even at 50watts i average 30-50ml a day. my main vape is custards but ive got at least 15-20 different makes so even if i vape custards all day ive never had vapours tongue in over 6 years.
i do change to fruits for a few mins every couple of hours and have at least 10 kits with a different flavour in as rotation is the key.
there have been a few posts lately about stained teeth with the majority saying they have never been affected. strangely enough i have and its only my front top and bottom that are affected but a quick brush and its gone. im wondering if toothpaste used can affect people differently. i use a high fluride prescription toothpaste. i think this is a case of no one knows the answer to this one as of yet.
also this may come in handy for future reference
POTV Vendors & Recommended E Liquid Suppliers