My primary thought was just, as a practical matter and to not seem croneystic, that the rank of Brigadier General only applied to the National Militia; but you would retain your primary commission within the US army.
In your normal duties you would remain a colonel, but when an emergency occurs and you must take command of the National Militia, you would have the relevant authority of a Brigadier General.
Does that make sense at all?
The National Militia would have to be a subsidiary of the US Army, otherwise it'd be some sort of presidential thug squad - that's not my style. Think US National Guard, not some sort of SNP-esque brownshirts organization. It's a home guard organization, not a political military auxiliary - if anyone is going to form such a group as the second it shouldn't have any official capacity, which is exactly why the SNP is going to be dissolved. States having official militias for internal matters is one thing, but any official interstate military bussiness is the business of the US army to coordinate - a group of states has no right to plan interstate military activities, it violates the division of powers in the constitution.
So yes, it would not be a civil paramilitary organization, instead think of it as a reserves corp or a homeguard unit, and because our limited officer corps can only be spared for such on a temporary part time rather than full time basis this also means that we have to conserve our talent better - which is the reason why most officers would, in any part time voluntary duties within the NM, usually carry a higher rank than their standard. We can't afford our top brass on homeguard coordinations, and its a good opportunity for the mid-echelons to be involved in grand strategy and central logistics in a less urgent reserves type setting.
Tl,dr: it's the national guard, not a new civilian agency with guns