>>96919958
Run Kingmaker with the players, run a few turns a week or so. Incorporate that into the rest of the rpg gameplay.
Other option would be using a less fantastical version aka defantasticalizing Worlds Without Numbers and giving each player a faction to use in the faction turns part of the campaign.

If what you are looking for is anything even a bit like osr, social mechanics aren't really useful. You're going to be better off reading Tony Bath's Ancient Wargame campaigns, getting players who give a fuck and putting together your own FRK sort of deal.

Kingmaker is an easy way to have a ruleset people will start negotiating and plotting about so you don't have to fuck around with making one that's going to cause inevitable disjunction with the rpg part.

Get use to mediating and making good judgments.

Its not a thing I care about much but Lion & Dragon is likely a useful resource for the rpg side. Strategic level operations is either boardgame subin, ACKS or An Echo Resounding/WwN.