Potentially you were the means that their product did succeed thanks to your talent, but assuming they were lucky to have you on board and you play that role it doesn't mean they wouldn't have done it without someone else.
So, what you should do I think is deal with the burnout, feel better and then when everything is clearer decide what you want to do.
About the salary, I suppose the market defines the salary except if you are the founder/co-founder you will always feel being exploited.
Potentially you were the means that their product did succeed thanks to your talent, but assuming they were lucky to have you on board and you play that role it doesn't mean they wouldn't have done it without someone else.
So, what you should do I think is deal with the burnout, feel better and then when everything is clearer decide what you want to do.
About the salary, I suppose the market defines the salary except if you are the founder/co-founder you will always feel being exploited.