The OP is in the UK so I am assuming a lot of those miles are in the wet, if that is the case, 6k miles is more than reasonable for a set of rims. Switching to discs is not a terrible idea though. If that is not an option, you are right with the Rhyno Lite being a great rim when durability is a must, I have a couple customers doing round the world tours on them without issue they are so strong. Another great option for dealing with quick rim wear is a rim like the Velocity NoBS, it does not have a machined sidewall so it takes a lot longer to wear it to a dangerous point and the extrusion is still very nice so your braking is still super smooth.