cracks

What slab are you removing. What is needed is a way to match the brick when you change the door unless you are planning a window the same size ,which BTW will involve a more expensive window. I think we need a bit more info and those cracks do not seem very bad to me I have a 1956 ranch with those types of cracks in a couple of places but they are not an issue I have been told. I think we need to see the rest of the outdoor space and also the indoor area where you want the window.

It appears that you have stucco above the patio door. You might consider a panel of that beneath the window once it is installed in place of the door. You will need to assess the threshold crack once the door is removed. Be prepared to have it replaced. Good luck!

I don't want to make things unstable if I remove the slab.

That slab is just a cap to cover the holes in the brick. You can remove it (carefully if you want to not damage the brick under it) with no fear.

Are going to make the window the same size as the door or shorten it up?