Tangential, but instead of making DC into a new state, why not just absorb it into Maryland? Virginia already absorbed the part of DC rhombus west of the Potomac.
National Democrats would also like two more Senate seats (and both DC and Maryland are predominantly Democratic, DC absurdly so) while national Republicans are happy enough with the status quo, giving DC no voting representation in Congress at all. So there's no strong momentum for movement on any side except DC's (who don't have the power to do anything about it by themselves).