For Mbappe, the best action to take would be joining Real Madrid this summer and it seems like he will. Real Madrid are his dream team and Real Madrid want to sign him so bad too. PSG wouldn't be able to do anything to keep him for at least one more season I think. Whether they win the Champions League title this season or not, Mbappe wouldn't care about it and still would join Real Madrid. And considering his future, I think that it is better than staying at an unpredictable team like PSG. Real Madrid are a far more promising team than them.
Yes, although there has been no official statement (be it from Real Madrid or Mbappe's side), it is very likely that Mbappe will join Real Madrid next season. Actually, this is a big loss for PSG, they squandered the big money (180 Million Euros) offered by Real Madrid last summer but in the end, they had to let Mbappe go to Real Madrid for free.
It's not meant to belittle Ligue 1, but La Liga does have a higher level than Ligue 1. So Mbappe's decision to move to Real Madrid is the right thing. In La Liga, Mbappe has the means to maximize his potential, plus he can achieve his dream of playing at the Club he idolizes.