Small FYI: JetBlue is fantastic w/instruments. Also have not had trouble on United or American. This is all with a cello. JetBlue notes: “The item is pre-boarded and deplaned after other customers have deplaned.”
But, often gate agents do NOT let you board early which is really no biggie. (Your numerical boarding status is a thing with airlines these last few years, due to carry-ons/basic fares.) But it is worth an ask. Be flexible. It will all work in the end. As someone above notes, just ask for the seatbelt extender as you board with a smile on your face and act like you do this every day of the week!
AND, place that instrument in the window seat - always.