First off, thank you for your service! And thank you for taking the time and effort to help others like yourself, as it is not only healing for them but also for yourself.
I would focus a little on what you want social media to do for you. You are not likely to reach homeless veterans, although many do still have phones and access to social media, so are you hoping to bring awareness to people or to get assistance from your community?
Think about what you want your social media to do for your cause, then think about the user journey for the people you want to reach. What matters to them, what stories will help your cause, what information can help.
There is a great book called Contagious and you don’t have to buy it as I know your watching your funds, but there are YouTube videos of the author talking about the concepts and I think it would be very valuable for what you are trying to do, which is tell stories that are shared and to get people emotionally invested in helping you out.
I am normally not a huge fan of Facebook, but for this community and effort, I would consider being on Facebook, as well as Instagram.
Lastly, hit your local VA, CAP, VFWs, and recruiters, to find ways you can get them to share your new organization and efforts, as well as to find out more information for all of the above.
Again, thank you for your effort and for your service.