Some good points, but how can a planet organize a strong defense if it has no planet to center the defense on and if the threat of planet destruction lies in the air. Do you think the common population would accept open resistance if the response is utter destruction?
True but the Star Wars universe has already shown that it is possible to organize a resistance without a centralized command structure that can be targeted and destroyed, making it it difficult or impossible to destroy the opposition.
The entire chain of command could be spread throughout space, on board command ships, space stations and so on as could manufacturing plants (factory ships), allowing these to be constantly mobile.
There would probably still be bases on planets such as recruitment and training centers (you can't avoid those) but those are only the end points of the command structure.
Decapitate them and new ones will pop up somewhere else, especially when such an attack also comes with punishing actions against the local population.
The First Order could continue to destroy planets, especially those who support resistance/the republic, but outside terrorizing civilians it would not get them anywhere. They are not destroying crucial installations such as command centers and shipyards and so on, the means to organize and wage resistance.
What the First Order would need is the means to detect any number of vessels or installations in space and a way to rapidly destroy these once a possible Republic military build up is detected.
If they can control space they are in a much better position to attack and control worlds.
Back in Star Wars IV Leia already commented to Grand Moff Tarkin that the more the Empire sought to control the galaxy through fear and oppression the more worlds would slip through their fingers. They simply can not be everywhere at once or monitor distant systems and sectors in real time.
Had for example the destruction of the current Republic capital been followed up by a massive conventional invasion (using shock and awe) the First Order would have done much more damage to the Republic's ability to recover and organize a military counter strike.
Perhaps not an apt comparison but it would be like Nazi Germany managing to cross the Atlantic undetected and bombing New York City of Washington DC, expecting that would break the United States' effort to wage war and strike back.
Instead it would only piss the US even more off and double their efforts to get back at Nazi Germany, all the Nazis would do is make it even more personal for the United States.