defund the police is a tag line that people are already behind and is fairly easy to understand
the problem isn't with the messaging, the problem is that people don't necessarily understand or care what is wrong with the current police institution. Change the tag line all you want, but you will still see many people who say they don't understand because they don't understand how large and systemic the problem is, or they just don't agree with the messaging at all. Any tag line you come up with is going to be heavily scrutinized, but diluting it by saying 'rebuild' or 'reform' or whatever, is only going to make it easier for politicians and the like to skirt around making actual systemic changes that provide real results.