In any of the runs that require you to go through the End Portal to end a timer, if you die right after you kill the Ender Dragon, would you still have to keep that timer running?

Well you said it yourself. Timer ends after entering the End Portal and the credits start playing, which doesn't happen when you die after you kill the dragon. The timer would indeed have to keep running after that.

Yea the run doesn’t count if u die in the end frame. That’s y Bedrocks better u can die in the end frame and still go through the portal

You have to actually go through the exit portal for the run to be counted, and if you die while the dragon is in its death animation, you will have to go back to the end and walk into the portal for the run to be completed.