The best answer to post #1 can be found in the engine manufacturers guidance, not what "some guy" at the airport or on the internet offers as an opinion. The manufacturer has spent some $$ on engineering and gets to hear lots of feedback from the field.

There is indeed a conflict between what a new builder needs to do in phase 1 to check the aircraft flight characteristics and identify issues vs what an engine needs to break in properly. I agree that performing a safe first flight is most important. But that does not conflict with the engine manufacturer's recommendation to minimize ground running of a new engine. Taxing around at low power serves more to build pilot confidence than to confirm that the ship steers correctly. That functional check can be done on the taxi to the first flight. And a quick control check immediately after liftoff from the runway should confirm that the controls are rigged correctly, or not, in which case shutting down and landing straight ahead is the best option. If good, go fly.

The actual result of the engine's cylinders and rings not breaking in is high oil consumption and then pulling the cylinders and rehoning them. Frustrating but not fatal. Not worth overthinking it before flight #1.

All of the manufacturers have their recommended engine breakin procedures on their websites.

Best of luck,

Wes