I first learnt Ga as always attached to Arimasen so still trying to grasp the nuances when changed to a Wa.
From my understanding of Wa vs Ga differences, it's just simply a matter of topic vs subject marker but I find it pretty interchangeable. Sometimes they use Wa and sometimes Ga without any particular important distinction to make were they to use the other. Eg. 俺を逆らう力はない｡
It feels like one of those things similar to desu node vs desu kara where one is supposedly focused on subject/topic when in reality people just focus on the formal/informal aspects and screw the subject/topic marker part for the most part.