Yakuza: Like a Dragon

To fully upgrade the Romance Workshop, you will need to donate 20 million Yen total. I would suggest waiting until later in the game to do this as it's easier to earn yen in the late & post game. If you're not doing it in the post game, you can use the Sotenbori Battle Arena to make money and grind EXP. You will pull in 1 million yen every time you finish floor 30. You will also acquire some gear that you can sell (The electric whip sells for 1mil and you can get it every run).

If you're doing it in the post game, which I would recommend, you can use the Millennium Tower Dungeon. The first boss fight will be with a Lv 70 Fake Saejima. He isn't very hard. When you beat him, it'll net you 5 million yen (7.5 million if you use a Sengoku Coffee). I'd suggest going back outside and running it over again (and again and again as long as you want). This is an easy and quick way to grind out money to pay off the Workshop.

Once you deposit the 20 million Yen, the achievement will pop.

Hope this helps!

The first guide is good, but if you want to earn money faster, the business management minigame is the way to go. I'm #1 and after every shareholders meeting, I get 3 million yen for the main game, and shareholders meetings are every 2-3 minutes, so you can rack up money quickly. While I don't remember the steps I took to get to this point (or how I got some of my employees), I can share what I currently have (my business makes around 200 million per run):

Ichiban Confections
Employees:
SR: Sojimaru (substory)
SSR: Bill Jobs (Need three personality traits to be level 8)
SSR: Maeda

Dianthus:
Employees:
SSR: Kamiyama
SR: Hikaru
SR: Da Silva

Stonefish
Employees:
SSR: Gonzaemon
UR: Eri Kamitaki (either get the business to #1 or do that to unlock her and get her bond to level 5, I forget which)
SR: Hosoyamada

Media Barter:
SR: Hanzawa
SSR: Techno Yuta
N: Omelette (start the minigame)

Grand Biu Marino:
Employees:
SSR: Professor Miyazato
SR: Lady
SSR: Micho Yamazaki

Industrial Hall Building:
Employees:
SSR: Director Shouzu
SR: Clara-chan (Substory)
SR: Redman

Notes: Make sure to upgrade your properties to max and take out loans (only as a last resort and even then I wouldn't recommend it) when you need to. You should level up your employees to compensate so you can maximize profits.

Here's a guide that shows which businesses to buy:

https://www.polygon.com/guides/21575452/yakuza-like-a-dragon...