We want something that we can put over existing drywall.
It's not pretty, but they make soundproofing sheets like drywall you can install. Can't paint it.
If you're subject to Home Owners' Association regulations, check with them, first.
You may have a clause in the agreement to deal with this.
Tape record noise, indicatind date and time of day/sound source, using a decibel meter as additional proof of sound intrusion to present to HOA, if possible.
Given the construction of many condos and apartments, sound intrusion is inevitable, particularly beneath upper owners' uncarpeted areas.
Soundproofing an Apartment: Non-Invasive Fixes for Quieter Renting