how to make it so that when a player with a custom effect drinks a bucket of milk, he dies?

You could make a procedure that detects when a player finishes using an item, and then check if the player has the custom effect, and if so then kill them.

If you want to make a custom death message, you can use the new Damage Type element in MCreator 2023.4.

If you want, i could send you either a picture of the procedure, or the procedure itself.