“Air Japan” is arm of ANA that operates some less “business-oriented” routes. It is probably what you booked. I’m pretty sure the operator has not changed on the final leg there. Before complaining about that leg, you probably want to be sure you just didn’t overlook it - which is common - it’s disclosed in fine print when booking. A change of a/c type to Japan from U.S. is not unheard of and can change once or twice in the 11 months a particular flight is available for booking.

if you want an exact type of plane to be on, best wait until within a month of departure, but even then it can change.

A note to management is useless, because what you’re complaining about is not unusual in today’s flying. It’s not limited to United. Sometimes flying can be overwhelming, but there probably isn’t anything in your perceived issues that are really problematic.

Air Japan is most likely what you booked in the first place and a/c changes happen all the time.