I don't think Trump cares much whether he is or is not impeached. He's a businessman. If the Dems make that move on the ol' 4D chessboard, he'll counter that move. If they make a different move, he'll counter that instead. It is refreshing to having a president who is not captive to traditional political thinking. And, to the limited extent the Dems realize their political antics aren't working, their alternative to traditional approaches is to go so hard left that they alienate nearly everyone.
If the Dems stay in their comfort zone, they look like a bunch of old, worn-out politicians. If they go hard left, they look like Stalin reincarnated. If they go toward the middle, they fail to differentiate themselves from the incumbent. The Dems have nowhere to go, and the whole "impeachment inquiry" displays that for all to see.
As you note, impeachment is just an accusation, so an "impeachment inquiry" is just a lot of hand-wringing about whether to think about considering the possibility of maybe making an accusation. That is how the Dems appeal to the voters that they should be entrusted to lead the nation.