When I read this in the original posters start of this thread:
"One thing that has stood out to me is how consistent these 5 lps are in sound quality."
I immediately suspected that the original recordings probably did not have a consistent sound, and that required the consistency to be a result of audio engineering after the "master tapes."
Let me be clear: I do not know if my suspicions are true!
But, to make a case, those records were recorded over a period from circa 1964-1970-something? During that period the explosion of record sales feed big changes in the recording studios. 2 and 4 track were common; was 4 track 'state of the art' in the early mid-60's. By 1970 had 8 track recording become more common?
There was probably also a learning curve by everyone involved, from musicians to studio engineers to equipment makers.
I remember reading that The Beatles after a few years and 3 or 4 or LPs told George Martin that they couldn't really hear the Ringo's drums, or bass drums. They thought they role in the recording process precluded them, the musicians reporting to the recording (was Martin the producer--whatever he was) that the sound quality of the recordings was not to their liking.
And we haven't even talked about mono vs . stereo vs bass before subwoofers. Add up dozens or hundreds of things like that, and I would expect any LP recorded in the UK (and America) in 1964 to sound different than one recorded 5 years later.
Sorry for the long rant.
And Keith Richards is wrong. Srgt Pepper is great.
