What are all the way you can increase earnings and what’s the most efficient one?

Almost everything in my opinion increases earnings,

Temporary earning increase can happen from just having someone like your village; you got money from the tax office in trade wars (temporary fix); cheeky hustling on the sidelines in your guilds (temporary fix).

For more permanent earning increase joining an active guild increases building earnings for all members providing the council Hall is full; increasing retainers apptitude and beauties intimacy and charm; marrying off heirs; deploying all retainers to shops; abit further in the game stores have mini game kind of things and upgrading them helps too. Don't forget to increasing your employees for each store. You also get instant card kind of things which are in your bag using those to upgrade your retainers and employees increase earnings. There's many many more but yeah...

Match your retainers to your beasts and don't forget to upgrade those guys too and release spare lower ones to get more beast eggs to upgrade.

Hope this helps.