I think the US invasion of afghanistan is a good example to refute your claims. Afghanistan was the culmination of decades of US efforts to control oil and gas reserves around the Caspian Sea, and get the ability to transport them without passing through Russia, Iran, or Turkey.
With this kind of tech at this period, they probably won't need a professional standing army. Even a small skirmisher made out of few picked men would be enough to plan fear to the enemy.
Behind a small group of skirmishers is a massive supply chain, a police force to resist blowback, the establishment of bases to house and defend everyone. While the US had troops on the ground, we also trained Afghans to police the territory on our behalf, and maintained expensive bases in Turkmenistan, Uzbekistan, and Bosnia. We deployed an equal force of contractors to fight, staff bases, and feed the troops.
Putting fear in the enemy also incites guerilla resistance. The taliban were once proxy soldiers armed to fight the USSR. Then we switched to arming the northern warlords, which turned on us. Many ended up joining the Taliban.
This rifle can easily assassinate enemy leaders from far away, whether it's during open field battle or from the castle, and without command chain the enemy would have a hard time doing anything.
In Afghanistan, instead of rifles, the US used Aerial Bombings. The Taliban organized into guerilla bands that even if we killed their leaders they still could launch attacks. Even after 20 years, once we withdrew the Taliban had the capacity to easily retake the country. Political blowback occurred as well; Afghanis did not like a foreign army dropping bombs on them (We killed more civilians than Taliban fighters to a large degree) nor did they like the mercenary contractors who acted like lawless thugs.
And it's not like ShizukOda need to direct everything by themselves. Making tributary states, whether by force submission or assisting the weaker sides in a conflict, is always a choice.
Like mentioned before, those tributary states are only loyal so far as we keep them fed. The Taliban turned on us, and the northern Afghan Warlords turned on us. The US provided billions in support to Turkmenistan and Uzbekistan, and spend billions to maintain bases in the region. It's expensive as hell to make tributary states and it causes expensive blowback.
Also, I think the "leveling the playing field" only really come to play as an issue later if we assume that the enemy doesn't stretch themselves thin, fighting against strong, numerous, and experienced invaders, and also trying to conduct experiments to figure how steel and this new gun work all at the same time.
But it always becomes an issue later, because it's easier to decode enemy tech than to invent your own. There's also the likelihood that tributary militaries will turn. The US trained and armed the taliban in the 80's, they became disagreeable in the 90s, and became the enemy in 2003. Then the US trained and armed the northern warlords, who were immediately disloyal to the US empire.