Somebody already asked you "why"? And, you never answered.

Which puzzles me.

You aren't against upgrading parts, the fact that you upgraded the brakes tells me that you aren't that worried about maintaining the "vintage" appearance. Freehubs and cassettes exist as an improvement to the threaded wheelhub and freewheel because the axle is supported by bearings located farther out on freehubs with cassettes. There are fewer axles failures (breaks and bending) on freehubs compared to thread hubs with freewheels (especially when considering 8-speed thread wheels and freewheels vs 8-speed freehubs and cassettes. And, the appearance of a freehub and cassette is just about the same as a threaded wheelhub and freewheel.

Did you ever take your relaced wheels back to the Builder for tuning after riding on them? New spokes need a break-in period, afterwhich the wheel needs to be trued/tuned, and the spokes should stabilize if they are quality parts, properly installed.