If your ELO plateaus is it realistic that you can ever be 500 or 1000 pts higher eventually?

Are books, courses, and puzzles really that helpful?

I'm amazed that GM's can memorize dozens or even thousands of games, or hundreds of openings/variations, which seems virtually impossible to me.

Depends. If you are able to break through the barrier somehow, the possibilities are endless. Also depends on the rating you get stuck at.

These resources are great, but it's essential to approach them in a right way. Regular, consistent practice with books, courses, and puzzles is key. Establish a routine that works for you, dedicating time each day or week to chess improvement. Actively engage with the material. Rather than passively reading or watching, try to solve problems, analyze positions, and apply what you've learned in your games.

@1

"Are books, courses, and puzzles really that helpful?"
++ No. Reaching 1500 is a matter of blunder checking before you move.

"I'm amazed that GM's can memorize dozens or even thousands of games"
++ It comes from experience. You can do that to. It is amazing blind people can read Braille.
It is amazing deaf people can read sign language.