Alvarez and Haaland complement Yamal's playstyle, idk why you are making this seem like one or the other. Yamal's output will increase with each passing season, on average, until it hits his usual peak level around 23-24 or so
Haaland is out of reach. Alvarez will be tough but not impossible. Kane seems like the most convenient option financially.
And no, Haaland won't be a foil for Yamal like Ferran is. Haaland doesn't make decoy runs. He goes for the kill and attacks the most dangerous areas to get goals centrally. Haaland mostly plays on the shoulder of the last defender and gets in behind. The Haaland and Yamal combination can work but Haaland won't be making the decoy runs people are talking about. It'll work but not in the manner they are describing. Yamal will be Haaland's new KDB. Kinda similar to Messi and Mbappe's chemistry in PSG.
The poster above makes a very good point actually. Ferran is the guy to do that. If you want a guy with intelligent movement who can be a foil for Yamal, Ferran is your guy.
Harry Kane and Yamal both prefer the ball to feet and Kane loves to drop deep so there might be some sort of tactical misfit there but Kane is still the most gettable option for you financially.
I'm not convinced that Alvarez can lead the line as Barca's CF under Flick. I think he lacks some attributes required to be a Flick striker. Might work under another coach but I don't think he's the answer for a Flick system tactically. Great player though.