My Precision is 8 1/2 lbs and neck dives. I purchased a strap from Pinegrove to counter this. The result? A nice bass and a nice strap.
https://www.pinegroveleather.com
I had this problem when switching from a J to a P.
I found it's all in the EQ - I keep the bass and treble at noon, bump the mids at 1k and 1.5k and adjust the tone knob to suit, normally 9/10.
Give them one last chance before walking.
Tell the band leader that if another rehearsal is cancelled short notice then you will walk. Your time is valuable and the band is supposed to be professional.