Best HS adjustment instructions ever are here:

https://www.spectrum-cycles.com/mechanical.php

Scroll down to middle of page.

Head tubes ovalize when there is loose play to level of banging. Definitely happens, not that common. Just as often a loose HS and some impacts will bow the steerer. Which can be harder to fix than the oval headtube. OTOH a very slightly, I mean slightly, loose HS is the one that will last more or less indefinitely. No such thing as preload on a HS. Any tightness, one degree of tightness, you have a head bearing about to fail. Most head bearings do fail in short order, but they can be ridden a long time brinneled without anyone noticing. Exact same HS, any brand or model, adjusted perfect lasts forever. Raleigh headsets that were less than a dollar BITD could last as long as a Campy. Really cheap HS with stamped cups give long service if cared for. Perfect doesn't that much happen and you don't have to beat yourself up if not achieved.

Ten thousand on a Tange sounds short to me but I have sure done worse. If getting that consistently whatever you are doing is darn close and leave it alone.

Poster who claims not to have seen dented headsets needs to ride more.

Giving up on cup and cone heads and using needle bearings/cartridge bearings is reasonable. There are still millions of cup and cone bearings out there and mechanics should know how.