The process doesn't dictate anything like that at all.
Speaker only becomes President if BOTH the President and VP are incapacitated at the same time.
If only one gets killed, goes senile, gets 25th etc... The VP becomes President, then makes an appointment which is approved or rejected by the Senate.
Pelosi could only be President if both the Pres. and VP are done in at the same time, or the he VP spot is vacant because the Senate never approved the VP, and the sitting President is done in, somehow.
Also, Congress doesn't have the power to start the 25th,. It has to be either the President themselves, like how it's been invoked a couple of times now for the President to be operated on so someone is officially in charge while they're out from anesthesia, or by the VP and cabinet, which is the more permanent version that hasn't been done yet. Congress, and specifically the House can only start Impeachment proceedings, but can't actually remove a President on their own, and the Senate can't start their own Impeachment,they can only confirm or deny the House's request. Neither can start the 25th, only the executive branch can.
Also, what makes you think AOC is ever going to sniff the speakers chair? There are plenty of Democratic Representatives who are just as, if not more, ambitious as her that are way way smarter than she is. Remember that the New York Democrats themselves were talking about gerrymandering her seat out of exisistance less than a year ago because she is an embarrassment to them. Hell Mad Maxine has a better shot at it than AOC ever will.
