You're right in that The Nice certainly did sound like a proto-ELP, especially on the likes of 'America'. TBH, if I didn't know and you told me it was ELP I'd believe you.
I can't for the life of me 'get' prog ie. ELP, Camel, Yes etc. and I have no problem with Pink Floyd (or even unfashionable 80s era Yes). I suppose you really had to be there at the time and associate those LPs with drugs smoked and blow jobs obtained.
Prog. Rock. Some of this stuff was labelled 'Symphonic Rock', which in a way, it was.
Long pieces, not all of it 'catchy' or memorable, but taken as a whole, a kind of musical journey which some find satisfying. A beginning, a middle and an end, with variations along the way and occasional 'highlights' along the way, and perhaps a shattering crescendo at the end.
I guess you could say in terms of the Rock/Pop genre that's pompous or pretentious, but a lot of people liked it.
Like me

. One of my fave bands are Yes, one of my fave albums is Tales From Topographic Oceans. Largely ridiculed, it worked for me. I was never on drugs (and not a blowjob in sight), I'd perhaps imbibed a pint or two, but it hit an emotional spot for me. It still does. Perhaps that's because it made a big impression in my formative years, but then that's always the case with music, any music, for most people.
For the record, I also liked (and like) The Beatles, Led Zep, some Soul, Motown, 10cc, Average White Band, Buddy Rich, to name but a very few.