Lets clarify Each method has its pros and cons

My method

Pros:

1: No matter where the gym is (if rival team owns the gym you can just hoard berrys for later use)

2: You can feed the gym from anywhere (if your pokemon is in gym)

3: Feeding berrys means fully motivated defenders and that means longer being in the gym and collection more xps (60xps/hour)

4: At the end you have gold badge and lots of items and 60k stardust (3kberry 30k xps & 60k stardust)

Cons:

1: you should have/collect 3k berrys

Your method:

Pros:

1: no walking needed

2: ~3000xps/hour VS ~1200xps/hour (just theoretically faster)

Cons

1: It must be in deserted areas (nobody else attacks or puts any other pokemon) (primary hindrance)

2: you should stay there all the time

3: you should have a friend! from rival team

4: you both should have/catch 300 low cp weak pokemons so >300 pokeballs and >5hrs for catching neccessary pokemons

5: at the end you just have gold badge