Hello guys (and gals)
Hoping I could pick your brains (not literally, unless you want me to donate them onto Nurgle, in which case hand me that knife)

. I've recently got back into the hobby and working on my painting skills. I love painting (and converting) models but feel my techniques and skills could do with some improvement, I'm not looking to start doing commissions (I seriously don't have the time to paint my own models let alone anyone else) but would love to start getting to a standard I could look to enter at Golden Demon (loooonnnnnng way off that level I know but hey, you gotta have a dream right?)
Generally I feel my problem is I'm stuck in a rut of painting miniatures that are quite bright, cartoony and colourful. I really want to get my
DA's looking dark, gritty - I love the atmosphere in the Codex and really want to bring that to life in my army. I'm still trying to get to grips with paint consistency - the Terminator took 3 coats to cover the undercoat (Dark grey Gesso) but I've still been told it's too thick and need to thin my paints more, but if I do they generally come out very watery and runs off the brush (had to strip my Librarian and start again twice because of this).
I also feel I may need to use retarders as the paints just seem to dry out way too quick (I mainly use
GW paints as these were bought for me originally - are other brands better for consistency, painting, value for money, etc?).
I've also ordered myself an airbrush kit, hopefully forcing myself to try a completely different way of painting may help get me out of this rut and try something different. I've loads of spare models I can test on, just need to find some techniques to try.
Anyway, any advice you fine people can provide will be greatly appreciated.