The best way to earn money is running the cabaret club. If you played Yakuza 0, you already know how to run it. For everyone else here is a breakdown and tips on running your club and beating the championship.
You should unlock this by the millionaire or paradise championship.
Invest your money early to speed this up and make it easier. You invest in partners(Gamers Hero had a great youtube guide I used) and girls(makeovers and recruitment). When you makeover your hostess max out 3/4 categories, the last will will have to take a hit. Recruit as many girls as you can early on to fill out your staff.
There are 4 tiers of hostess, platinum, gold, silver, bronze. You want platinum or gold. You unlock more platinum hostess as you go through the championship.
There are 2 sets of skills you want to unlock before you start,club hustler and hostess with the mostess. Hustler increases the amount of money you earn each time and hostess increases the xp they get at the end.
While running the club pair the customer with the girl that will make him the happiest(the happier the guest the more they spend).
When the girls ask for help you have to pick the right option and you will get a bonus order. They make hand signals to relate the information. Here is a list of the solutions.
imaginary shot glass-ladies glass
open a book-menu
cup hands together-ashtray
upside down surfer sign-ice
There is no time limit on these and the timer stops so don't panic and pick the wrong one.
Some guests may cause trouble, noise problem is the most common. Never had to learn these(besides noise). As long as you have a good staff and keep the customers happy, this should not be a problem.
When you work the girls too much, they get tired. While tired their skills take a hit. once they hit neutral I give them a break.
Once you earn enough cash you can activate fever mode. It has 3 tiers, I usually use it on the first or second one. This puts random tables in fever mode(customers spend a lot of money). Try to use this when all of your tables are new so it does not get wasted on someone leaving in 3 seconds.
You can also get guests in to fever mode by getting them to extend their session. Only ask average or above to extend. More wealthy they are the more likely they will extend.
Lastly is oil barons, they have a chance to spawn at the end in the higher championships. You have to mash X to fill up a bar and he will spend a small fortune. The better girls you have the easier this will be.