I’ve done similar before and it works pretty well. Create two tracks - one for the stage recording and the other just the instrumental track (mono would be best, but not required). Align them as close as possible by ear so you don’t hear a delay on the instruments. Then find a section where there is an instrumental section with no vocal and also a loud-ish percussion sound (if one exists). What you want to find is a section that has a waveform that is visually distinctive. Then in the project window set the height of both tracks large so you can easily see the waves on each track. Zoom way in and visually align the waves by moving one track until the percussive elements match up. Now phase reverse the instrumental track. You’ll probably hear a drop in the instruments. Select the instrumental wave object and jot down where its start position is shown on the info line. Using the info line change the start position earlier and later moving by the smallest amount possible. What you want to do is find the spot where the instruments are the most canceled out - this can be tedious and take awhile (or not if lucky). As you move closer to this perfect alignment spot the instrument volume will decrease until you reach it and then as you move beyond it the volume will start to increase again. Next move the fader on the instrument track up and down until the instruments are most canceled out. If this fader is too low you won’t cancel some of the instruments on the stage recording, and if it is too high you you’ll hear it instead of the instruments on the stage recording. Since the stage recording will include the reflected sounds of all the instruments as they bounced around the theater you will always have some residual sounds, but you should be able to vastly reduce them. You might also find that using some EQ at this point, both on the individual tracks and the mix, also improves things. Take notes as you go so if things get messed up you can return to your last-best-settings.