Looking for a 40 (or 48 hole) 126mm rear freewheel hub with cartridge bearings
I believe that freehubs became mainstream long enough before cartridge bearings became the default for hubs such that I think the only cartridge-bearing rear hubs you'll find with a threaded interface will be for singlespeed freewheels or fixed sprockets, and none will have 126mm OLD width.

Since it sounds like you are a utilitarian (rather than leisure) cyclist, the best value for money would be to sell the nice 6-speed blocks to vintage enthusiasts and get a cassette hub with cartridge bearings. Cassettes start from 7-speed, but if you're using an unindexed shifter you should be fine.