Prepare for the aftermath

It seems that your prediction of your soldiers getting involved with the local people has come to fruition. Since the involved soldiers would be married off to the widows, they will be more receptive to be stationed at the new settlement as part of a local detachment.

But, even as other people have yet to understand what is actually going on, you're already moving on to the second stage of your plan. The upcoming marriages would make marrying the foreigners more acceptable to the tribe. Thus, more parents would be open to offer their daughters' hand in marriage to the soldiers stationed at the settlement.

While this is what you want, this must happen without local bachelors being left out. As you have learned from history, wars have been fought over a single pussy. So if you're going to make marrying foreigners a thing, you must get the local men to do the same as well.

It will be a slow and ongoing process, but the first thing you will do is to recruit locals to replace those soldiers that will stay. They will be thoroughly trained to behave in acceptable manner in time for your return trip to the capital. Once there, you will come up with some sort of accommodation for the recruits since they're not from the capital. Preferably, large enough to host a family even though they're bachelors.

Then maybe you can implement a reform program involving people from the slum. The tribal recruits can help them in that regard, and if 'something' happens, you're sure that you can deal with it.

After a while, some of the recruits would accompany you back to the settlement. Maybe few of those with big egos will brag to their friends and siblings that they're tasting foreigners' pussies, sowing jealousy among those who live at the settlement. More and more young men would then travel seeking foreign women to be their wives even though their own women are as good, or in certain aspects, even better.