Out of DC's other IPs, the Fourth World has the closest ties to Superman, definitely. But tying them too close together has been detrimental to both at times. The Fourth World saga seizes to be its own thing, and Superman's world isn't viewed as being strong enough without the Kirby additions (Bruce Timm's reasoning for bringing them into STAS). Also, while building the cinematic universe in which Thanos had already been established as coming and DC/WB were already getting criticisms for ripping off Marvel no matter what they did, having the League vs. Apokokolips be the thing that brings them together seems to be an easily avoidable source of comparisons. Hold off on Darkseid and co for a while, at least until the brand is strong enough that comparisons to Thanos won't be an automatic loss. The Legion of Doom is a more distinct trope that DC could have seized on and made their own before the MCU did it.
And lol at Jimmy Olsen being a foundational character and then getting his brains blown out in these movies. He should be there in some capacity, but I doubt the early issues with Jimmy, the Newsboys and opposing science factories had much appeal to Snyder.