This ...
In a 4 piece Classic Rock/Country cover band, all members are over 50. The singer is female, lead is a female, she formed the band and acts as boss
... doesn't equate to:
I wait a few days and send a group message to her and ask, what is your priority, I then say that not playing for 6 months prior to last gig has really caused me to lose interest in the band and I fear that her not having time to play now will really kill my interest in band. I just ask for a simple answer, what is first priority?
I then send a group text to the old lead player on the old band chat. Told her you’ve had your chance, been dismissive of me one too many times, I’ve taken the band to a new amazing lead player, see ya later.
Clearly, she doesn't "act as boss"; not based upon what you've shared here.
First off, you sent a
GROUP message. From what I can tell, you were instigating this. There were no private discussions with the singer or drummer to see if they felt similarly to you? No private discussion with the lead player, voicing your concerns and giving her a chance to either explain herself or to make it right with you guys?
This kind of sounds like a classic, passive/aggressive, power-play taker over.
Then, after a replacement was found, you sent another
GROUP message where you seem to be taking a bit of a vindictive posture (highlighted in blue) and belittling her, publicly. In the military, we had an saying: "Praise in public. Punish in private." you seem to have gone out of your way to do the opposite.
Lastly, you mention that
YOU've taken the band ...
Based upon what you wrote,
YOU seem to be "act(ing) as boss".
ETA: I apologize. I didn't answer the question in the title, directly.
1) I would have had a side (private) discussion with the lead player. I would have been brutally honest ("You're not even the best guitar player in this town ..."), but I would have made sure she knew why I felt the way I did.
2) I would have had a private discussion with the drummer to see if he agreed with my musical assessment. I would have asked him if he was okay with my divulging details of the conversation with the singer.
3) I would have had a private conversation with the singer and "taken her temperature" on the situation, possibly telling her about the drummer's comments.
4) I would have had a conversation with the drummer and singer, preferably face-to-face.
5) I would have asked for a group meeting and let the lead player know where everyone stood.