1) Quality comes in various sizes: mainly, the idea of the plugin (how useful is it) and the code itself. I believe that if the balance is right, you can come up with good quality. Obviously, you can’t do a WooCommerce-scaled plugin all by yourself, but if you have a small idea that is revolutionairy enough, you can.
2) In my opinion, most plugins require knowledge of JS and PHP. Not necessarily expert knowledge, but the more you know about that, the more you can do, and the better your code quality would be.
3) You don’t have to be an expert on JS and PHP — you kinda just have to figure out the parts you need. Personally, this book helped me a lot, because it taught me the exact parts of JS and PHP that mattered for a plugin: https://www.amazon.ca/WordPress-Development-Cookbook-Yannick-Lefebvre/dp/1849517681
4) See above
5) Not sure, but it does go into how to make your plugin more secure
Hope that helps!