For this I do not know how to unlock Ex Hard without downloading a completed save file.

However, using the same method as Yakuza 0, Kiwami and 5, you can unlock premium adventure and new game plus -- just not extra hard mode. This is most likely due to the fact that the hex value changed to 01 there only checks to see if you have completed the game, while this is enough to unlock Legend mode in later games, in 3 and 4 you must complete the game on hard mode to unlock Ex Hard.

So you need Download this save [drive.google.com] replace with your save (path - C:\Program Files\Steam\userdata\{user_id}\1105500\remote) complete the game and start New Game Pllus.
First Save - You need to complete last mission(mb its got NG+ from start).
Second Save - Legend difficulty start.

To lock your achievment back use SAM[github.com]