Okay, so there's a fair amount to get through here!
So let's start with the learning curve, and which way to progress. For the sake of ease I'm not going to talk specifically about airbrushing here, as that's almost a discipline of its own. I'm also not going to mention enamels or oils, because the ways they can be used are very varied, so they fall across a variety of difficulty levels.
In short, there is no 'correct' way.
There are always things that a beginner will have to master first; thinning paints, applying layers of block colour, brush control, drybrushing etc. These are the absolute first steps.
Then you come to slightly more difficult beginner techniques, like highlighting through layers or edge, shading through layers or wash.
Once your 'basic' techniques are fairly solid you'll want to look to doing things quicker (perhaps by under-painting) or smoother (feathering or glazing). These are perhaps intermediate techniques, but once you have a grasp on them you can definitely achieve a perfectly acceptable 'good tabletop' standard. Many (perhaps most) painters will stay at this level if they reach it - even some of those that can or could paint to a higher standard will choose not to for the most part, perhaps saving it for character or centrepiece models.
Following that there are a huge number of techniques you can learn, but no order in which you have to do it, or even a requirement to
If you're interested in triple-loaded-brush-offhand-blindfold-wet-blending
(God I hope that isn't a thing!) then go and learn how to do it, there's nothing stopping you.
Similarly, if you feel that you would be better served by learning the technical side of things (and this is something you should probably do at least a little bit of sooner rather than later, ideally before you even get to this step!) then brushing up (hah!) on your colour theory is there.
One thing you could do is find a model that you like the look of, and see what techniques have been used to achieve the finished product. If it turns out it's the Sky-Earth-Non-Metallic-Metals that caught your eye then find some videos on beginner SENMM and see if it works for you. The 'Eavy Metal style isn't the only one out there.
From what you've shown us, you know how to use a brush, and you can definitely colour within the lines, so to speak.
You've already suggested that you want to try out enamels. TRFT (YouTube link below) would be a good place to start there. Many of the videos deal with vehicles, but the techniques he's using can often be applied to infantry-sized models too - ask @James
You've also been trying out freehand recently - perhaps do some research into the kind of brushes best suited to it (without looking myself I would be suggesting sable-hair with shorter bristles for added control), and if there's any exercises you can do as practise.
One thing to remember though is that even if you have mastered the basics with every new thing you try you're having to start from scratch. You already know yourself that just because you can paint neatly that doesn't mean that freehanding a small design is easy! And the first time you try Object Source Lighting there's every possibility that it's going to look awful, but you'll still be learning.
Much like with painting your first models, when you wet-blend for the hundredth time it will look a lot better and be a lot easier than the first!
All the top painters will say this and its something of a cliché, but it's true: you have to find your own way. Take the techniques that you like, and forget about the ones that you don't.
So that's the long bit kinda covered, so let's move on to who I personally watch. I'll keep this a little shorter! Here are some of my YT subs:
; their channel focuses on what you can do with their (predominantly dry-)brushes. Stunning results with minimum effort.
; a multi-Golden Demon winning painter, who has a huge number of videos about pretty much every aspect of painting. He manages to make even very difficult techniques sound and look very simple, even for a painter of no real skill like myself. He also has some fantastic videos talking in depth about individual colours.
; he likes oils, enamels, pastels, spray cans and cheesy video effects, and he's very good with all of them!
; Adam talks in his videos rather than paints, but he has videos discussing lots of different areas of the hobby and how to approach it. The video list is massive but there are some absolute gems in there.
; a bit of an odd one here. This guy is fairly new to the hobby and his painting is (by his own admission) not that great, but he creates some fantastic results using almost nothing but leftover sprues! I love the fact that he does something a bit different, and is willing to have a go at just about anything.
; I'll probably never make a gaming table, but watching Luke doing it and explaining the processes behind it is a real joy.
The Race For Terra
; our own @Lovecraft0110
. Weathering. All the weathering!
Inspiration for me will be the shortest section. I can't stand Instagram, and I don't use Pinterest. If I want to look at models I head to YouTube, message boards and Twitter, though not necessarily in that order.
So I feel like this is a little rushed despite the size, and I'll probably have to come back and make huge edits, but I hope it helps a little. =]