Well, I finally gave up. I've tried numerous things to get v29 to provide the large icon sizes at 4K that I had with v28 at 4K, but no luck at all.
If I leave W10 compatibility for high DPI off, the icons are tiny. If I turn it on and select use this setting(...) + use scaling provided by Application, the icons are tiny. If I select use this setting(...) + scaling provided by System, the size is just like with v28, but dialogs are fuzzy as expected. If I select use this setting(...) + scaling provided by System Enhanced, the size is just like with v28, but I see lots of visual artifacting. I'm running an RTX 4080 with latest drivers and excellent system stability.
Finally, I uninstalled v29 and reinstalled the latest v28 TCMD. It looks great at 4K. I wish I could get v29 to look as nice as v28 (crispy-sharp resolution and larger icons), but I'm happy with what v28 provides. Heck, I started with 4DOS and NDOS thirty years ago, and v28 is a million miles past that.