Potty training in an apartment? Help

I am getting a male mini poodle at 9 weeks soon. We live in an apartment on the 3rd floor with a small balcony. I am getting nervous about potty training, and am just looking for some advice for the best option. Keep in mind, we will be moving into a house with a garden in 3 months time.

I work from home, which will help during this period. My concern is that whichever route I choose, I want it to hopefully be easier to adapt when we move to a house and they will need to go outside in the garden. What option should I do for the first few months in an apartment:

  1. Get a grass patch and put it in our second bathroom (which we don't use). Keep the door closed, and train the dog to go to the door when they need it. Open and allow it to potty on the grass patch in the second bathroom. This hopefully gets them used to doing it on grass, but do you think that it will be an issue down the line as they will get used to only doing it "inside"? We will obviously take it for walks too after 12 weeks when they can go out. But the first few weeks will just be inside on the bathroom grass patch.
  2. Grass patch on the balcony. My worry is the balcony is very small and has gaps in the railing. I don't want to risk anything.
  3. Puppy pads inside.
  4. Take it downstairs every time. I don't see this as being realistic if it needs to go every 1 to 2 hours.