I agree with your overall point that guns don't stop repressive regimes, but I think some of this analysis is a bit flawed.
First off, one can already see the difference that armed resistance can make in the continued existence of the various ethnic insurgencies around the country and the fact that they control significant chunks of territory. Without armed resistance, they would also likely be under the Tatmadaw jackboot by now.
Second, the US and Myanmar aren't particularly comparable. Democratic instutions aren't well-developed, the military has been the de-facto leader for decades etc. I feel like your argument might be better served by comparison to, for example, Nazi Germany. Contrary to popular right-wing arguments, the Nazis didn't take away guns. Guns actually became easier to get for those who were part of the in group. The thing is, those guns weren't used by those in groups to defend the out groups while the Holocaust happened through the gradual removal of rights from those groups.
One could make some comparisons there considering the "I need my guns to fight tyranny" crowd ended up joining the police in opposing protestors and supported Trump's slide into fascism.
Finally, there was this: "The only way that an armed citizenry could counteract a military like the Tatmadaw is if they were enough of an equal match that it could make the armed forces hesitate to initiate the violence, in the first place."
Like I mentioned before, an armed citizenry is already counteracting the Tatmadaw and has for decades: the armed ethnic insurgencies control significant chunks of territory. History is full of examples of underdogs carrying out successful assymetrical warfare campaigns. The Vietnamese defeated the Americans. The Taliban/mujahideen fought off both the USSR and the United States. Equal firepower isn't really necessary for a win.
I guess I take issue with your negative view of armed resistance against fascists. The Tatmadaw will kill you whether you are armed or not. They straight-up threw babies into house fires (that they set) during the Rohingya Genocide. These people meet peaceful protest with massacres as a rule.
I understand the point you're trying to make and I agree with it, but I'm just not sure this is the best example to illustrate it.
If anything, the Tatmadaw situation should steer the conversation towards the abolition of police and military forces that have amassed such an overwhelming capability for violence against the public which they also tend to use pretty regularly against said public (certainly the police). Chipping away at these instutions also chips away at the "we need guns because tyranny" argument.