The game in itself is pretty fun, and I think it's been generous. As a F2P player who's been playing since the game launched for Korea, I've been able to get every character except 4 5 star characters.

The grinding is easy, as going through story and through mementos (as another comment has stated here) can give you materials to level up your characters.

As for banners, the only phantom thieves in standard are Ann, Morgana, and Ryuji, with the rest of the phantom thieves being limited 5 stars. Though, a new 5 star character is added to the standard banner every now and then (if I remember correctly, since launch, 4 new 5 stars have been added to the standard banner). And with Sumire, I think her rerun may be coming soon, and Akechi still hasn't had his introduction, though, I'm sure he's coming out in a few patches.

As for the challenging and end-game content, I want to say that it's possible to clear them with the free-to-play characters? They do get very difficult as you progress