Thank you to all who responded. Thankfully it sounds as if I'm not off-base on this schedule. Much of this stuff is material that I supposedly learned as a child/teenager, but at that time I didn't appreciate it and so didn't apply myself very thoroughly. I didn't take music theory in college (my degree was in another field, even though I was heavily involved in music throughout), and I found that I have glaring holes in my knowledge and experience after the G5 level or so. Indeed, I had a bit of a mental block regarding four-part harmony until this year (due to the circumstances and material with which my former teacher tried to teach it - not her fault, she did the best with what she had to work with).
Victoria, LMusTCL is indeed an option, but as with LRSM in performance I would have to give it some thought. I'll also have to see what the set works are when the time comes. I looked at the 2017 practice test, and found the set works utterly and completely uninteresting. (Mahler's 4th Symphony, Brahms Piano Quintet... not quite my cup of tea.) But the set works for 2019-2020 are much more to my taste (Beethoven's Fidelio and Debussy's La Mer).
Crock, I don't know about my experience... Most of my experience is in other genres (specifically American Old Time, Bluegrass, and Irish Trad) that have limited application to the "Classical" world. However, preparing for the G6 theory has shown the value of my experience with those other genres. I learned how music works in a practical sense. I learned how to harmonize a melody and insert non-chord notes and ornamentation in real-time, while playing with others. (This would be on a guitar, banjo, or fiddle.) I have called those years "wasted" and "mis-spent" in the past, but they are really helping me now with G6!
corenfa, let me tell you the dirty truth about people with PhDs: we're not smarter, we just have a high tolerance for school. But you are absolutely right that decades of music experience help with this stuff. And you're also right about the synergy - the more I think about music theory, the more I notice in the music I'm preparing for ARSM. For instance, Beethoven's use of diminished seventh chords, Chopin's inversions and modulations, the brilliant four-part harmony in the Bach Prelude and Fugue... and I also find that the more I understand the music, the easier it is for me to memorize it. All this stuff works together.
I just wish I had done this years ago. Music exams were not available in (or anywhere near) the middle-of-nowhere Appalachian town I grew up in. I was only hazily aware of them when I was teaching in Alabama. When I started teaching here in Texas (where music exams are popular among some segments of the population) they were available to me (as a teaching tool and for my own use) for the first time. I thought about taking exams for years, and finally took the plunge once a student came to me wanting to take them.
ABRSM has been great for me, I must say. It gave me the motivation to get off my butt and recover my piano skills (I was about G8 level when I drifted off to traditional music as a teenager), and then push onward.