Wonderful Plays' channel has several videos where he is playing in 5-player and 6-player videos. There is good strategy advice sprinkled within those videos.
Back when I was playing a lot, I had decent results in 5p and 6p games. Compared to 3p/4p, these games are inherently more messy, and it is harder to come up with concrete guidelines. But a few ideas to keep in mind:
- The players across from you (there are 2 of them) are potential allies. If they go red and you go red, you can sandwich the players in between, and earn some easy military points. I like to keep this in mind when thinking of which cards to pass. For example, I would sometimes pass a red card that my neighbour cannot build but the player following the neighbour can.
- What you don't want is to get stuck in a red corner of the board (where both you and your neighbour are going heavy on red). This is usually disastrous for everyone on your side of the board, and excellent for the player opposite you. The easiest way to lose in 5p/6p is to be stuck in a red corner. By contrast, it is not such a disaster in 4p to be fighting for red with your neighbour, since every other player is hurt by the war.
- Green seems more viable in 5p/6p than 4p.