I'm pretty new to speedrunning and I want some tips as to how to get better times. One of the main things is how can I do the BLJ on the stairs before the 50-star door reliably, I can never do in less than like 5 tries.

Its impossible to do bljs reliably. Either you mash or you try timing it (when Mario touches the stair). I think there are enough tutorials for the rest :)

I recently learned something that may help you with that BLJ. When you get the first part of the blj which puts you up the stairs in front of the door but you don't think you're going to have mario go all the way through the door you can continue to hold up and keep longjumping against the door/wall. I've only started doing this recently but it seems to be the same type of method that people use for the 1 star basement blj by continuing to jump against the wall to gain speed. TL;DR -Get a shitty looking blj that doesn't seem like it will shoot you out the door -Continue holding up and longjumping -You might gain more speed and go through the door anyway