Just because a street is one way does not mean it is so narrow that only one vehicle can fit through.

Streets may be one way because it discourages unnecessary automotive traffic and it allows the rest of the space to be dedicated to bicyclist or use for residential and commercial purposes (outdoor seating, space for games, etc).

So even if the streets are one way, there will often be space for the ambulance to maneuver around blocked vehicles. And if there's not, the ambulance can take another street or road. If it's the only route that can access that location, then with any sane amount of urban planning then the ambulance will be within a short walking distance to the location (and more often than not within eyesight).

Prohibit on-street parking on at least one side of the road, make the side walk on that side big enough that an ambulance can use it to pass other vehicles if the road is blocked (I'll skip the question of why the street is blocked in the first place). Actually enforce the parking restriction.

You need space for pedestrians anyways, and with traffic restricted to the necessary minimum you don't need bollards everywhere

This isn't some kind of rocket science that has never been tried before, it's the reality in many cities around the world. You need an entire concept instead of adding half-backed solutions to an existing system, but the solutions are all well-tried and for the most part very straight forward.