I was actually working this morning with one of my trombonist who does the same thing. I told him to think of the release more than the attack and showed him the concept by using a handclap that is an attack and then one focused on a rebound release - it helped that we are in a very live room this year thanks to covid - and he could hear the rebound vs the attack. I'm woodwind also but this approach has helped many of my brass over the years. I also told him to practice playing a scale with 8 8ths per pitch, focusing on a release of the note by the tongue rather than a thrust forward of the tongue and to make sure each note was centered in the bullseye and focused the same.
