If you are looking just to see the ending because the grind is too much for you, here it is, it took me a long time but I was able to do it alongside with conquering the world so it wasn't as bad as I make it sound

The most important prize is EVIL Hatsune Miku whom you can marry at 1500 Notoriety. She is a unique NPC with a unique portrait you can't use for editing. Her stats sits at the low 80s for int and mid 70s for charisma, so she is basically an eye candy officer who can only help you do more slanders (Her instigate 3 is amazing for battles though) She is also one of the needed wives for the love for beauties achievement.

The main feature "Loot" may seem attractive at first, but the problem is that it costs 100 action point to use each time, you don't loot all that much, it doesn't work half the time and most importantly it only raises Notoriety by 10. Like I guess I can see looting kind of useful in the beginning of the game like how it was in the original ROTK8 but now Koei is stupid enough to make the bar of entry even further by requiring you to have 1000 Notoriety to loot in the first place.

The TRUE advantage of Notoriety is what you actually gain Notoriety with, which is Slander. This allows you to lower the loyalty of enemy officers without the need to use precious strategy points every parliament, the only catch is that you have to do it yourself so make sure you have high int and deception ready!

Other than that, you get some quick bucks from Tavern jobs, occasionally repeating Tales quests and events such as Bandits and Guards randomly giving you money, pocket change like 100 mind you, it would be OK at the start of the game but by the time they start giving you these loose change you would already be rather rich.

The fastest way is to go to another city not owned by your faction and go to the palace, select slander and start slandering away! You get 10 each time for 30 AP

The other even faster way but you don't get that until late in the game is the 100 AP Tavern Quests that yields significant notoriety boosts, those become commonplace once you are high enough and you gain 50 each time you do it, it's more efficient for sure but you do need to actually find the job and be high enough notoriety for it

There is also another fast one, which is to rebel against your current ruler, that will yield 300. I hear that if you are a vagrant army and cause a governor to rebel by slandering and joining said rebellion as a mercenary you gain 300 from that as well (But I haven't confirmed that part myself)

Looting sucks, 100 AP for only 10 and you have a chance to get hurt, not worthwhile at all

The Notorious tales story gets unlocked around 500 and 1000 Notoriety, they give 10 to 20 points each to complete and are repeatable, but they are far less efficient to slandering

Overthrowing the Han Emperor gives you like 50, that's how low the Han has fallen.

Killing and Executing Generals, even famous ones like Guan Yu gives you no Notoriety, which is kind of stupid

In other words, getting 5000 is stupid slow and you shouldn't bother unless you want achievements