There are a lot of threads on this. Usually it has apartment or neighbors in the thread title. I don't think any of them really came to a solution.
Maybe the best...
Find out what the legal requirements are for what you're allowed -- If you're allowed to practice between 9am-10pm, nobody can do anything to stop you.
Work out an agreement with the neighbors.
Buy a digitial piano and use that whenever. Then use the acoustic when you're able.
I don't think anything short of physically sound-isolating the room -- building a room inside a room -- or buying a movable practice room would really work. You can pad all over -- the walls, under the piano, the ceiling, etc., but sound will still get through. There are places that make practice rooms that an upright would fit in -- probably a grand too if you want to pay for it, but if a little one costs $3-5,000 I can't imagine what one for a grand would cost.
It sounds like you've got being on positive terms with the neighbors. That's good. Get them to tell you if there's a problem instead of going to the managment.
I don't know if anyone's ever mentioned building or buying a practice room on here before. That is one solution but it's very expensive. Otherwise there really isn't a solution for eliminating the sound. Not that I've heard of and not that I've seen on here.