Here is something I accidentally found out. Did you know pressing all 5 or 7 keys at the same time as fast as you can or in rhythm will get you an S rank every time and sometimes an SS rank! This wont be necessary to complete every song with an SS rank just a nice little secret.

Heres the main guide.

Go to the free dlc girl "The sexy secretary girl" and complete her first or second story, this will unlock a song that pays 1000 gold every time you play it and it only cost 300 gold to auto play it which you can have running in the background on mute while you do other things online. This is a 700 gold profit.

In this game both the songs and the story mode have an auto play option that give you a perfect SS rank every time and getting all girls to max love level 4 is a simple matter of having enough money to to buy them gifts.

Now that you have all the money in the game every achievement and H sex scene can be unlocked as fast as you want them to be.

Have fun!