We upgraded to this wheel on two bikes. Brompton New CNC Rear Fender Mudguard Wheel ABEC 7 Bearing Multi S | eBay

This wheel has a rubber ring as its rolling surface so not hard. It is much better than the original but I have not yet decided if it is enough for my use or if i want the two wheel thing, a "half rack" or full (ti) rack.

I think the OP need to decide what a rack would be for. If it was rolling only a rack is maybe a bit heavy. keyven said early in the thread "- the extendable wheel addon is AMAZING for stable rolling (haven't found better) - a must imo". Looks like he used one and is happy with it.

There are also lighter options both in ti and carbon. Looking at them myself but can not find any info on what they can carry (when used as a rack). They are light but expensive.