Glad you're onboard the Perspective Tool v2 hypetrain. It helped me understand a lot of things and it's an incredible help when making perspective corrections.
Something Marc does not mention in his videos is that any composition that has the horizon in the shot is unlikely to benefit from a 3rd vanishing point. In fact, 2P or 4P is the way to go in those cases - his own demo has huge distortions in areas from ignoring this soft rule.
Use 3P perspective for anything that is vaguely isometric, but still converges. No horizon in the shot. Think Diablo III's angle, where things converge but not too much, like the brickwork and pillars on this bridge. This is where 3P feels strongest.
You are correct that lining a sketch will make it go away from the painterly look. You can still use your perspective guides to inform you on how to correct a sketch. Values are the way to go for landscapes!
Great work, keep it up!