Why do you think the sign was ever intended to unite people?

If you wanted to try to suggest an improvement in that regard, you would need to understand the motivation behind each of the lines; despite some pertinent criticism, you've missed the point in a number of cases (on purpose in some cases, one would presume).

For instance, it is inarguably the case that "black lives matter" is a divisive slogan, because you can simply observe: did this unite people or divide them? Well, united some, divided others. And the implication is exclusive because only one group is mentioned. Overall: divisive. It's a criticism of the status quo, though--wouldn't you expect a criticism to be divisive?

Now, if you wanted a unifying sign, "all lives matter" is no better, because it completely ignores what prompted "black lives matter" to begin with. You don't unite people by ignoring their concerns.

There are a variety of ways to regain a more unifying message--though maybe at the expensive of brevity or crispness. Any of these would do:

"Black lives matter because all lives matter."

"Don't black lives matter?" (A legislatively actionable subtitle might be: Police Reform Now.)

"Black lives matter too"

and so on.

(If the issue is specifically police brutality, there are also unifying slogans one could devise around that theme that aren't necessarily racially phrased: there is plenty of unnecessary police brutality against people of all races, and if we solve that problem, then any racial disparity in treatment would be less acute of a problem.)

And on it goes.

So I don't think you've done a very good job at your stated goal. Maybe you don't actually care--maybe the point here is fake-inclusiveness, showing that you too can write a superficially "inclusive" sign that really is as much about needling the "other side" as anything else. Or maybe you do care, but if you do, you need to remember basic rules about how to get along with people.

Most notably, people want their concerns acknowledged, not dismissed.