The First Order havent fully revealed themselves to the galaxy, and its in the Resistance’s best interest to destroy them before they deploy. The oscillator is the power system for the base and destroying it would blow up the base, which would cripple the First Order. Maybe the mining of the planet could be used to imply that the planet’s the source of all the Star Destroyers also.
Perhaps the scene where the Hosnian system is destroyed could be recontectualized to be an attack from a First Order Dreadnaught.
How about have the Dreadnought be the Supremacy instead?
That would probably open another plothole about why couldn’t the Supremacy just obliterate the Resistance fleet from a distance (assuming the Supremacy in this version would have weaponry similar to what a Mandator Dreadnought’s). The original chase sequence in TLJ is already headscratching enough (did the First Order run off of dreadnoughts?), it might be better to not muddy the waters any further.
The Mandator Dreadnought destroying the Republic HQ could be more feasible and leave less questions hanging.