This is minor, but I can’t help it. Before the briefing at the rebel base, is it ever established that Finn was stationed at the Starkiller?
It just seems to come out of nowhere at the end of the movie that in addition to being this hero, he also knows that the weakness to the thermal oscillator is in sector 45 or whatever.
Also it’s super convenient and coincidental that the first order just happens to discover the location of the rebel base right before this big battle by tracking one of their scout ships.