one suggestion is to become involved in workshops, conferences, summer programs, and colloquia. if you can pull off organizing one or more of these things, then that’s even better.
of course, maybe you’ve been there, done that. i mention it because networking outside your institution is one way to boost your odds of success in this chancy process. this is because name recognition - not only your letter writers’, but also your own - can be a deciding factor in surviving the first cut. if someone on one or more adcomms has a positive impression of you before even reading your app, this could push you to surprising heights despite a patchy UG transcript, unexceptional GRE, etc.
anyway, it sounds like you’ve come far - well done!