Hello and welcome to the forums!
Yup, training your pup to go potty on command will help in the generalization process. If you use it early on, your pup will associate going potty with the cue and that should help with going potty initially in a designated area and then expanding to other places.
It's always best to check with the vet to determine how safe/unsafe it would be to use the outdoor potty area (infectious diseases are more endemic in some areas) and what's the best protocol based on that. The vet can also inform you on how long you should avoid certain places and also can help provide options so not to miss the critical window of socialization (which closes around 12 to 16 weeks).
However, the fact that you'll need to walk across the street and it's next to a busy road makes it rather inconvenient (on top of potentially unsafe since he's so young). At 8 weeks, puppies may need to be taken out as often as every 15 to 30 minutes when active, and by the time you would put on the collar, leash, open the door, then look both ways before crossing would likely be too late and your pup wouldn't make it on time to get to the designated area.
I like the idea of the grass patch, especially considering that the large grass area across the street is going to be long-term potty area once he's better potty trained. The fact that the surface area looks similar helps a lot. I feel though that, if you could, a patch made out of real grass may be the best option. Grass patches made of real grass require that you purchase a customized tray and then routinely replace the grass every 1 to 4 weeks depending on the frequency the pad is being used.
While this is likely more costly, the biggest advantage is that dogs are naturally drawn to the smell of natural grass, and on top of this, the real grass does an excellent job in absorbing liquids reducing odors naturally. There is therefore not much need for cleaning or maintenance since the entire unit can be fully disposed of and replaced with a new one on an as-needed basis. Fake grass instead tends to get smelly without daily cleaning, does smell like fresh grass and any waste that is not timely removed can easily stink up a small space.
Other tips that come to mind:
-Learn to recognize the signs your puppy needs to go potty. As a you recognize them, entice your puppy to go to the grass patch and be ready to praise and reward when he does.
-Monitor your puppy carefully, with time, he may start going on the grass pad on their own. Be ready to praise enthusiastically and give several treats in a row when this happens.
-If your dog has accidents around the home, use a paper towel to clean it up and transfer some scent of his urine on the grass pad.
-Clean all accidents around the home with an odor neutralizer like Nature's Miracle.
-Make sure the balcony isn't located in an area with scary sights or sounds as a puppy who gets spooked will feel less safe to use the potty.
-Don't give the full run of the house, you need to keep your puppy in a small area of the home where you can always supervise him and he can readily see and access the patch.
-Consider that some dogs dislike going potty on the balcony when it's cold out, windy or when it's raining.
Hope this helps,
Make sure to post pictures of your new Doxie once he joins your family!
Best,
Adrienne