I guess I didn't totally get the Mercerism and animal angle, it seemed to exist mostly to highlight a difference in empathy between characters. I think the film is better without it.
The android test Mexican standoff was amusing though.
I probably just have some bias here since I like the film's nuanced interpretation of the androids and having one of them show mercy at the last moment is a better twist to me.
I was a bit skeptic of Philip K Dick once I finished it but I'm glad I took up Ubik a couple of years after - it turned out to be one of my favorite reads.
Another good PKD read is A Maze of Death... it has very dated tech in it (satellite in orbit playing pre-recorded tape)... but the ideas are very similar to Ubik (in a way). A world deteriorating, an almost spiritual war between forces of order and disorder.
'Ubik' the novel, not the substance, I take it. in the novel [spoiler], Ubik is a store-bought aerosol product which can be used to temporarily reverse deterioration... or is it...?
I don't think these are spoilers, but stop reading if you are sensitive: Ones that stuck out for me were "why is this weird guy living in a building that is falling apart?" The story reveals that Earth is basically poisoned and that everyone who can leave has left, and the people staying behind are suffering serious cognitive decline due to it.
I'm afraid I disagree with your claim that J.F. is "suffering serious cognitive decline" due to his life on Earth. To the contrary, his confinement on Earth is not the cause of his psysical decline, his physical condition (genetics) is the cause of his confinement.
more details: https://bladerunner.fandom.com/wiki/J.F._Sebastian
The film differs in that J.F. Sebastian is a creative genius, not sub-par IQ, and yes his genetic condition is the proximate cause of his confinement on Earth: but the film clearly asks: what caused the condition? The film doesn't say but pretty much implies the same as PKD's novel [that one of Sebastian's parents or ancestors was radioactively damaged].
In general the film implies but does not lay out any of the (unnecessary and seriously tiresome) exposition from matchstick characters that PKD's novel did, and that makes it stronger. (Arguably except for Gaff, and especially the unicorn scene in the Director's Cut). Ditto, omitting Mercerism in place of a general existential malaise.
And I believe one of the things Deckard talked to his wife about was saving up enough to buy a real sheep, which is what leads him to the Tyrell corporation.