yes you're missing two very important things.

1. use your imp. improved imp.

2. make a macro for drinking. you'll be using that button often. very often.

3. there is a way to kinda delay a spell cast through macros, but its not automatic and requires several button clicks. by using empty castsequence commands you can acheive this.
/castsequence , , , , soul fire

you're going to have to test it out to see how many commas you will need in that macro to make your soul fire cast when you want it to.