Entering with a completely clean phone is suspicious. To any border patrol agent intent on abusing their power, that screams "I'm trying to hide something and want to be detained for an unspecified amount of time".
If you're trying to protect your data, probably better to set up a secondary, plaubile profile, and restore from backup after crossing the border. Or to take a burner phone and buy a completely new one inside the US after crossing.
Maybe it's suspicious in Silicon Valley, but I know plenty of people who make minimal use of their phone. In Japan, flip phones are supposed to still be fairly popular.
If you're trying to protect your data, probably better to set up a secondary, plaubile profile, and restore from backup after crossing the border. Or to take a burner phone and buy a completely new one inside the US after crossing.