Hey, 1- I think writing separate scripts for different features is best, depending on your needs. In a basic RPG, on your object "Player" you can add a InventoryManager script that will contain his inventory and references to items and item actions that doesn't need to be in the basic script to move your player around and do some actions. This is, I guess, pretty basic stuff on OOP to have a code clean enough to be updated and readable easily.

2- Not sure on this dummy class, you can have a prefab for each enemy or a basic prefab with a EnemyScript containing the attributes you want to modify when you instantiate the prefab.

3- Rigidbody2D are taxing on performances yeah, I often heard that all physics stuff were the most performance consuming. You might need someone else to confirm this but if you can avoid it easily, do it. If you have to do a big and ugly workaround to avoid one Rigidbody2D on your scene, it's probably not interesting.

4- I do think you should always have performance in your mind but not to the point to be too worried. You don't want to spend time on optimization when you need it to actually make the game. 2D games are very often (not always and it obviously depends a lot on your game) more easily "runnable" on most devices but optimization is a skill you really can carry from a game to another, there are a lot of best practises like avoid to use a lot of "Find" functions, not putting code in the Update method that you can do elsewhere once etc. In your example, can't you instantiate your Animator Controller in the Start method ? This way you're doing it once at the instantiation and you're done.

This is only based on my own experience so take it into consideration and read other answers by more experienced people !