I have been speed running on vanilla for a while now, and I think its time to find a mod. I don't know where to start looking, and I am away from home right now at school so I cant really watch YouTube videos. Is there any speed run mod that I can customize to stop the timer after a certain random task is completed, such as killing the warden or filling up my inventory? If there are multiple, what would you suggest.

Speedrun IGT https://redlime.github.io/SpeedRunIGT/ has support for multiple categories. It is probably the best timer for Minecraft speed running so I would recommend it.

list of mods here: https://www.minecraftspeedrunning.com/public-resources/mods also you prob should get multimc if you don't already have it. Really great launcher!