I'd say he was foreshadowed more than "a bit", as Dany's entire arc with the Warlocks was basically a precursor to his appearance and his role in her life. The reason why he wasn't introduced before Tywin Lannister is pretty simple IMO; Tywin was the purely human/political big bad of the first part of the story and he had to be the big thing the Starks, Baratheons, and even his own children had to struggle against.
But we're now at the point where things are shifting to the magical and Euron is a reflection of that, he's somewhat familiar in the sense that he's a Greyjoy from a familiar place in Westeros, but he's clearly connected to a lot of supernatural goings on and will be important for the story as it moves from the transition stage of human v human, into human v undead. Everything in the books points to him being the last great challenge for Dany and Tyrion and most everyone below the Neck and in the East. That's a massive character, not just "Oh hey my long lost uncle is back". It was akin to this thing coming from across the sea with lovecraftian intentions for some of these characters that we have, as you so eloquently stated, "followed for 3000 pages".
And the way I see it, Euron in his show depiction was getting towards the Sand Snake area. You have a character in the books who talks of madness and horror and his megalomaniacal schemes for the world, yet in the show he's a guy who couldn't stop making dick jokes and spouts off lines like "Let's go kill my niece and nephew." It's such a poor depiction, especially when the major aspect of the character is how utterly unlike the Ironborn he is. Their culture is based on apathy towards the suffering of others, whereas Euron is a sadist who deliberately revels in it. He's basically Balon 2.0 here, and it's a wonder why they didn't just make use of Patrick Malahide or alternatively cut the plotline out completely from their version of the show (as it was clear they intended going by Season 4)
It's just a waste, that's what I'm saying. Its a character that has weight and relevance in the books and could have been used beautifully if the show wanted really go there. But they didn't fully commit to the character or the storyline, so we've now got something that honestly feels like an added wrinkle. I mean you're saying that you hated all of the newer material introduced in the 4th and 5th books, well this is basically the same thing only without the proper weight that it has in the book format. The show would have been much better if it just stuck to its guns and went ahead with the streamlined, Daario navy they introduced in Season 4. We've only got something like 17 episodes left to tie things up and this isn't helping things.