Knowing theory is a huge help, but funny thing is, some people learn it the clinical technical way (that helps sharing) and others figure it out intuitively, without a clue as to the names of the notes or the clinical names for the various chords. I'm one of those who has squat for intuition, so learning theory really improves my playing.
I use a tuner app on my tablet. Yes, if the instrument drifts in tune, get another instrument. I've ditched a few that do weird things over a short period of time or under stress (bending strings).
And yes, proper setup makes a huge difference. Not just the action, but also the intonation.
I'll look you up on YouTube. I'm scared to google myself.
Well I need to be more like you. YOu should perform.