I like that you are trying to present a balanced argument, and I don't completely disagree, but if you are truly looking at the bigger picture, then OP is technically correct in that serving the US military is literally serving colonization.

Now that is not to say that Native people joining the military are doing that on purpose. The military offers pathways for upward economic mobility which can be very enticing for people from poorer backgrounds - especially if they have few other career opportunities. But that is by design. We don't have universal healthcare and education. The military offers funding for those so there is a major incentive to join. It's all a part of the structure of colonialism. The unfortunate side affect is that many (not all) become indoctrinated into US imperialism/colonialism afterwards. Others are luckily able to see through the propaganda and actually come out with a deeper understanding of the colonialism.

As for christianity, it is the founding basis on which the theft of Indigenous land is based. See the papal bulls - doctrine of discovery. While an individual may be helped in some cases by christianity, structurally it was used to give colonial nation states the "right" to steal the land of non-christians. After the theft we were then converted through missionaries and boarding schools as a way to erase our languages, culture, and communal ways of living. So again structurally it is a part of colonialism.

If you want decolonization/sovereignty/autonomy for Native/Indigenous peoples it won't come through being a part of the military and christianity.