i'm not knowledgeable enough about the detailed german history in this timeframe but german military were literally STRIPPED by the allies in addition to suffering heavy casualties during the great war, being flanked in two fronts, verdun, etc.
also many military guys actually sympathize a lot with the right wingers (DNVP and the rising Nazis), so maybe their paramilitary wings might also have access to those fancy military toys but yeah i agree, logically if you can have the formal military guys on your side it should be easier to win the war (not only you get their manpower, but the reichsbanner would also LOGICALLY benefits from close relstionship with the army, with them providing training, intelligence, access to fancy equipments, potentially weaking the right's connection to the army, etc).
And yes i also personally think at least decent relationship with the army should be crucial in deciding who wins the civil war, after all the army is not only about manpower but also connections andthe key to politicking with guns. For now imo it's silly as hell how much easier it is to militarize the reichsbanner into basically a maoist militia, lol.