Most things are simple. If you need them turn them on. If you don't need them, turn them off.
IPv6 doesn't "screw things up". If your internal network has devices that can use IPv6, and there is some advantage to it, then, if properly configured, it will have no impact on your IPv4 structure. LAN vs WAN is completely separate so it doesn't really matter, from the LAN side, if your ISP supports, requires or offers IPv6. Typically though, most home / internal networks are quite happy (and simpler) without IPv6 (at least internally).
The same strategy applies to pretty much every setting. It is a worthwhile exercise to go through the settings one at a time, particularly after a major firmware update, and determine if a setting is something that applies to your setup or not. Every network is different and there is no right answer, or the option wouldn't be there.
If you run across something that you don't know what it is, google it, search on here and then post a question if you are still unsure. There are literally hundreds of options so a list would be worthless and trying to discuss every setting in a single thread would be a nightmare.