There came a point (fairly early on) when the FKG devs sat down and decided what direction to go in: i.e. whether hardcore grind was necessary to get an income. They chose "no" for the answer and the free event girls have been trivial to max out since then - min/maxers may complain they're not amazing in function but there's some pretty good waifus you get for free. The 5* girls who enter the gacha during an event get a spotlight and it's ~50% chance per 11 rolls (i.e. 50 stones) to get a 5*. Provided you are selective with who you gun for, it should be reasonable to get a 5* waifu without paying. 6*? Hope you're willing to pay when the special deals come in.
There is an EXP bonus to feeding on-element manyus. The dragons have to be on-element when it's time to evolve.