you say it couldn't be all the things you listed in bold but it could quite easily be a number of they things mate, you can't write anything off,
in the pic with the cotton looking almost burnt in the coil but fresh on the ends, is there a bit too much cotton packed in tightly with the coil ? i have found if the cotton is in there too tight that happens to me, i appreciate you have said you have 100's of wicking videos but there really is a fine line between too much and too little cotton
personally 9 times out of 10 any issues I've had with flavour, or vapour production, burning cotton,spitback etc is all down to the wicking process
this morning for example my new serpent mini RTA was struggling to wick and after 3 or 4 draws i could taste a wee dry burn in the background, i fixed it by literally cutting a ml of cotton of either end as it was just a tiny bit too much in the channel
i would also say if you have only used 1 type of coil, (1 manufacturer) it's worth trying a different coil setup, i know you say you don't have time to make coils but a simple single wire NI80 3mm int diameter 0.5 guage coil would take less than 1 minute to build,( i use that as a suggestion as that is what i currently have in both my wasp nano and Hadaly)
i have used pre built coils from different places (still to try
@Smut coils but i will be) and i have found the best results are from the coils I've done myself, it's easy to use cheaper poorly made prebuilt efforts and that usually tells when you vape on them
buy yourself a wasp nano rda which is single coil, get yourself a roll of Nichrome or NI200 or Kanthal wire and try a simple build which is easy to wick and i almost guarantee unless you are having issues with your tastebud's the flavour and vapour production will be spot on,
it also could be the juice you haven't said what type of juice you are using.
I'm just thinking out loud here but as i say don't write anything off and go back to basics on everything and slowly ensure every part of your set up is spot on
hope there's something in amongst my ramble that potentially helps, I'm relatively new to building but i have spent a lot of time making mistakes and learning from them and i think the most important thing to appreciate is, you will always be learning new things, sometimes you will think its a perfect build and you vape and it tastes like crap and you have to tweak things here and there
another thing when you install your coils before you wick them are you giving them a fire to make sure there is an even glow? eliminating hot spots etc ? Ive forgot to do that plenty of times in a rush to vape and it messes up the whole thing
anyway I've typed enough and like i say hopefully something in there that helps