The question was not what the “BLM movement” should do, but what an individual Americans should do; your steps do not seem actionable for individuals. Your steps 1 and 2 also partly beg the question.
Additionally, assuming the support of all Democratic politicians is highly dubious; a number of cities that have been marked by highly visible abusive police behavior in recent weeks are already controlled by Democratic mayors and city councils, who in many cases have nonetheless refused to hold the police accountable. And support of 50% of the population (which BLM now has) is certainly not always enough to pass “whatever policy you want” absent coordinated organization and in the face of political inertia; for example marijuana legalization has had majority nationwide support for years but has no near-term prospect of passage at the federal level.
No? I want to help BLM achieve its goals, but “launch a nationwide discussion” and “come to a consensus policy” are not actions I can personally take. If I post policy proposals on Facebook it seems unlikely to me that many people will read or be influenced by them; it also seems unlikely that they would be better than many other policy ideas already out there. If you actually do think that lack of policy ideas is the most important bottleneck for BLM and that personal Facebook posts by non-experts is a promising way of addressing it then that’s a possible answer, but if so I’d like to see your analysis for why you believe that.
Note that at the national level this is inherently very difficult because for any proposal made by one party, the other party has an incentive to oppose it in order to deny the proposing party a victory (and the accompanying halo of strength and efficacy). But fortunately this is not necessarily a problem for at least some approaches to the police reform issue, because police are mostly controlled by state & city governments, and as noted many states and cities are under undisputed Democratic Party control, so the relevant politics are within rather than between parties.
This seems to have already been done; reports of looting have become increasingly rare and polls report public sympathy for BLM is very high.