I'm so glad you're enjoying learning all of this man! You can see that you're motivated to know it
I'll go point by point:
- Spot on
- Add chords can add: 9, 11, 13 - This is because most chords in general are built from the R 3 & 5 - So if you added them again the chord quality wouldn't change. Like a G guitar chord has more than 1 Root (I'd call it that instead of 1) & 3rd. The 9, 11, 13 are colour tones, hence why they can be added. You don't get 'add7' because a C chord with an added 7th is Cmajor 7 - this would be a Major 7th chord.
- Extended chords are all in the name, you're extending the intervals after the octave, 9th, 11th, 13th etc
- Yep! Minor chord spelling is R b3 5 - So it would be C Eb G - D# would be enharmonically incorrect as it would be considered a #2, which isn't how you're using it.
' Diminished flattens the 3rd & 5th - C Eb Gb - You're using a D# scale here - I'd reccomend thinking of it as Eb instead - this is because Eb is easy to spell & D# is a Theoretical key (not the most practical to spell/use)
Eb: Eb F G Ab Bb C D - see how it's alphabetical, no doubles note types like F, F#?
D# E# F## (yes double sharp!) G# A# B# C## - hopefully this proves my point
I would look into that Rowan J Parker Major Scale memorization video & the construction of a Major Scale to really get why G isn't the 5th but it's to do with the amount of semitone, tones you construct a Major Scale with.
- Love Maj7 chords! Theres no difference between a Cmaj7 and a Cadd7 apart from the fact you'll never hear it called 'add7'
If you add a 7 to a Major chord, your turning it into a Maj7.
- Minor 7 chords are spelled R b3 5 b7 - So spot on!
- Spot on! Dom7 is R 3 5 b7 - The combination of the 3 5 & b7 gives it that dominant quality. The power chord is just called a power chord because it's got the Root & 5th in it - two.really stable tones that don't conform to Maj, Min or Dom7 because there's no 3rd or 7th so it has the 'power' to take on all e forms technically. And it just sounds powerful, hence why it's used it Rock & Metal.
- Spot on with Augmented too man, well done!
A cool trick with Aug triads is that they are Symmetrical - meaning any note can be treated as the Root! So for every Aug chord you know, you really know 3.
CEG# - C Augmented ,
E G# B# (really C) - E augmented
G# B# (really C) D## (Really E) - G# Augmented
It's the same for Diminished 7th chords - R b3 b5 bb7 - (yep double flat)
The main thing is that you understand how all of this stuff sounds so you can hear a chord and be like 'That's Augmented' - the Theory should always label what you hear
Hopefully this is some more to chew on!