It sounds like you've had a hard time finding a wheelbuilder familiar with heavily loaded wheels. The rear wheel of a Dutch-style bike carries about 60-65% of the static load with just a cyclist on board. It'll carry roughly 100% of any additional load on the rear rack. Compared to a racing bike, it's also much less likely that the rear wheel gets unloaded for bumps, potholes, and general road roughness.
Does your bike have a rear drum/roller brake? Those options put additional load on the spokes that your casual wheelbuilder might not consider.
On the other hand, your rear hub is most likely symmetric, which helps make a more durable wheel.
That said, it sounds like you need more durable wheels. I build a fair number of wheels for tandems, which see a similar level of abuse. There are some components that work really well: stiff rims, brass nipples, heavy-duty butted spokes. Proper tensioning and relieving is important, too. Which rear hub(s) have you been using? Which rims?