You are right. Harry Kane is just another option, but Real Madrid is serious to sign him if Tottenham sets a proper price. Unfortunately, Tottenham responds the interest of Real Madrid with an outrageous price for Harry Kane. Recently, I only heard the rumor Bayern wants to sign him, Real Madrid seems no longer negotiating with Tottenham. Sure, Mbappe price should be lower than previous price because his contract is almost over. Mbappe also doesn't want to stay in PSG, so PSG management must respect him by setting a proper price for him. However, since Mbappe is a superstar, it is always complicated to sign him. Real Madrid must do more big efforts to ensure PSG management to let him go.
Actually, there is information that Real Madrid and Paris Saint-Germain have already agreed on a deal under the scheme of 224 + 56 million euros. This is certainly not the 300 million that PSG initially seemed to have requested, but it seems to me that the club should be quite satisfied. In theory, Mbappe, on the contrary, might want to stay at PSG for another year to get his loyalty bonus of 90 million euros, but still, it seems that the presentation of the player on July 5 or 6 should take place soon.