Glad you’re enjoying it so far! I definitely made the game tougher, though some parts later on might seem unfair since I didn’t test the lategame as much as the earlygame.
As for the 6 characters, 4 of them join automatically. The other 2 only show up if you have a certain amount of gold. You’ll need 10000 Gold at the start of Ephraim’s Chapter 12 and 8000 Gold at Eirika’s Chapter 12. It seems kind of steep, but you get a white gem from Innes and L’Arachel, so it shouldn’t be too hard.