It would be really nice to not have the lanes mixed up in a lot of situations. Is there such a checkbox somewhere, mod-wide or entity based, doesn't really matter? I haven't found one, but I'm not that good at looking. If not, would this be a worthwhile feature request?

Agreed, I'd very much enjoy the ability to not have all items round-robin. I'd even enjoy a way to make long lines and only have certain items go to certain belts, I know this would require more stress to be put on the item, but I'd deal with it if were a config option to enable.