Organic and non-Poppish Progressive Metal?

That's not the issue. https://en.wikipedia.org/wiki/Passive-aggressive_behavior

You also reject recommendations for surprising reasons. "Meh - the cover art is too colorful". I mean, you could have stated up-front that you're really picky with cover art. But of course that would lead to practically zero recommendations, since nothing really fits your rigid criteria which nobody can predict. That makes it almost appear like your asking for recommendations only to then be able to criticise them.

Most of the non-"synthetic" prog metal artists fall into prog-death or prog-thrash territory, the majority of which use harsh vocals

Also, I find it interesting that you don't want maudlin or sentimental vocals, but then put Lateralus on your list. Don't get me wrong, I love Lateralus, but it's pretty poppy/alternative-leaning for prog metal 

Or some have not understood your initial post. 

Non-poppish?! What does that even mean? What kind of vocals are those?! Catchy vocals does not equal pop vocals. And what kind of pop vocals? Classic pop? Modern pop? 

Fairly clean?  Clean vocals but not that clean? 

Does Pain Of Salvation qualify? They got an amazing vocalist, and their songwriting I find excellent. 

Riverside has catchy vocals, and yet you listed an album of theirs. 

Yes, it's about the kind I'm looking for, and it's also about to become one of the greatest Prog Metal classics, so it's very good to include it here. I can thus certainly understand your fondness of this album too, I'm afraid though that personally, I don't find it to be that good or particularly original. - But I may give it some more spins and see what I then think. It would be a nice addition to my Metal collection, even I'm not very fond of the flashy and very colourful artwork, and can't get it fit with the music. 

^ Frankly speaking, the band name alone is too silly to me. 

Edit:

Anyway, it inspired me to give King Gizzard a new listen with another album, and this one too.