If the attack on ~~Coruscant~~ Hosnian Prime is too challenging, perhaps a more feasible solution involve no move against the Republic City at all. Instead, the First Order fleet could attack D`Qar?

There is already footage from TLJ. You could explain away any “charging” mentions as the Dreadnought.

Poe and Black Squadron wouldn’t go to Starkiller, although they would be prepping for an attack when the enemy fleet arrives. Instead the opening for TLJ will play alongside the conclusion to TFA. Han’s detonate charges cause the area around the base to explode, which is why Rey and Kylo are separated. It doesn’t destroy the entire planet.

After the destruction of the Dreadnought the First Order retreats. Then the Falcon returns to D’Qar and everybody celebrates for a short while.

This also permits TLJ to start at an unspecified time, as our first scene with the Resistance would instead be them in hyperspace. Poe wouldn’t be demoted although he can still have his character arc- the moment the Supremacy shows up he says he wants to jump in a cockpit and blow something up.