Reading article from "https://unreal.fandom.com/wiki/Unreal_Tournament_2004_Single_player", I'm noticed interesting thing for SP players like me.

"And if that isn't enough, there's a hidden ladder included in the game, more exactly an Onslaught ladder. In order to unlock it, the following lines must be added to the file UT2004.ini and a new profile must be created in order to get access to it.

[XGame.UT2K4LadderInfo]
AdditionalLadders=Class'XGame.CLOnslaught'

"Additional" tab will show up after completing existing ladders and the Boss map. Upon clicking "Additional" tab, a blank page will be shown and "Onslaught" tab will appear below other tabs. By clicking it you will select Onslaught ladder, where you complete the maps similarly to any other ladder, except nothing will be unlocked after completion. Tested on v3369 32-bit (Windows).

Tip: If you do not want to complete the whole game just to unlock additional ladder, use SkipMatch cheat code."

I'm not trying this yet, but SP ladder lack of famous "new game type", and Onslaught only available as Skirmish | Multiplayer mode.

Hope this help some one.