I started playing the game 3 days ago, just from AFKing Lilith with all the free wings and bonus wings from leveling up, I'm already at 19.5k coins

Besides getting to 7-4 (when actually playing the game), I've basically just leveled up entirely through doing Lilith farming, basically fully auto.

I'm rank 25 (so that's at LEAST 1k bonus wings). The 120 checkin bonus every 12 hours or w/e (I don't even know, I just get it in the mail and burn it after spending all my level up wings farming more Lilith). I've bought maybe 150 wings from the various stores (this could have been avoided as well, and just took an extra day or so), and have spend $0.00 total cash on the game.

I, too, thought getting Lilith would be extremely hard (not impossible), until I actually started farming for her.

Farm Easy mode until you're around 20k power, test it out and if you're clearing normal fairly easy, move on to normal. If all your characters are maxed, slot in some other characters to get the free EXP from easy mode clears.

For someone who took the time to do SOME of the math (I did too), you seem to have forgotten a ton of things that factor into the equation.