Am I able to record a twitch highlight of my speedrun, then put it into my computer and then time it?

If you want to add an external timer to the video that's fine, but the in game file select timer is the only accepted timing method for console runs so you would not be allowed to submit anything that deviates from that.

By in game file, do you mean the Elden Ring game time? Sorry if it sounds stupid.

You need to show the final time at the end of your run, which can be seen on the "Load Game" screen. On console, you have to quitout after the credits and then open "Load Game" to show the time.

Ok, thanks man.