You can just run past them if you care - the very first time I simply drove off, when I came back to S rank it I had all the gear I could possibly need.
If you stand in the entrance of the castle, you can place yourself so they are at the stairs and only there head is visible - after that it's mather of patience.
Hardest mission by far is the metalic archea on extreme, god that was annoying, followed by that one that has {buddy spoilers}
You fight off a billbion vehicles with quiet. Had to cheese it by placing mines on the one road you can place em on before it starts.
Cash is no issue at all once you relize that you can just sell resources to max out your gmp. Select Premium metals, hit left arrow, gmp caps out at 500 000 000 - ish, sell, lose maybe 17% of rare metals.
I now have all these lovly tools - but for the "fast" S rank, without farming objectives or point enhancers, I try (often fail) to for the no shot fired bonus. Don't use any toy, don't fire a single shot, leave no evidence and the S rank is certain.
Many of the missions don't feel S rank worthy, since you can win em so easily if you know them, littarly shoting a rocket into a window or sprinting to where you know the prisoner will be.
Also D-Dog forever. His tactical fulton is the best.