Interesting opening but I think you must start from the premis that the purpose of the Borg is the assimilation of life forms, not wholesale destruction.
The Borg fights if you resist, but even during a confrontation they assimilate new recruits.
I personnally don't think planetary destruction as a revenge tactic, it would violate their basic programming.
Please note in STNG First Contact, the scene where ther was a futuristic look at a Borg colonized Earth.
Recommend you try investigating all the post Borg collapse of all the planetary systems under their control after the Borg Queen was destroyed.
Immagine a post Borg civilization trying to reinvent itself, even to the point of trying to learn how to breed!