Beck wants to become the new version of the Avengers, he wants to displace them and gain the power and glory that comes with it. He mainly wants the recognition and the power for it... fame and glory, that’s pretty much it.
They also have a smaller goal during the film which is that they want to gain control of their inventions again by coercing Peter into giving Beck the EDITH glasses. That appears to be a lot of their main initial reasonings. However, they are also brought together through their joint hatred for Tony Stark. So they are then trying to get people to forget about Tony and his sacrifice and worship Mysterio instead. Displace his memory because they don’t want him remembered so they need someone new to take that place.
That is part of the reason why Beck works with Peter to get him to go back to a normal life and why he reveals him at the end. Peter is the new Iron Man and his protégée so getting him to give it up further removes Iron Man from people’s memories.
Quentin Beck: You’re right, you may not be ready, but this is my responsibility.
Spider-Man: Far From Home
Lastly, you ask how would he defend against a real threat? Well assuming he would try and fight it depending on its nature the illusions would be very effective. Remember how effective they were against Peter initially.
Also if the plan went to plan he would have had full control of EDITH and the Stark security network. There’s massive power to that, that he would be able to leverage to fight threats. The illusion tech also has a lot of extra functions that could be useful here and there.