Gave my notice. Manager is planning to negotiate better terms to convince me to stay. What should I do?

Now I've ALWAYS believed that if your employer is only willing to give you a raise/promotion after you threatened to leave, then you're better off leaving. Though, I want to make sure that I'm not blinded by tunnel vision so here I am...

Situation:

Have been working at great company for 2 years. Last month, they cut hours and salaries due to COVID.

Then, I got headhunted by a Fortune500 company offering me 20k more and a specialist role, meaning great career growth in a more niche industry. The new team seems to fit my personality type exactly, too! So of course I'm out of here, right??

While giving notice, I found out that my immediate superior, let's call him M, also quit the same day... SO now the manager wants to discuss what it would take for me to stay, given that the team's whole support team will be gone.

I've been eyeing M's position for 2 years! I now see a chance to take this position, and have amazing leverage to negotiate for a hefty raise (more than 20k). If they offer me M's position, it would also be great career growth, and demonstrate 2 promotions over 3 years. BUT also due to personality differences, working for this corporate team has completely eroded my self-esteem and every day is a grind.. I'll get paid more, but also assume more responsibility over a team that I don't feel fulfilled to support. I'm only close to one team member, but he begged me to stay and I'd feel terrible about leaving the team at a time like this.

Any thoughts before I speak to the manager? Thank you!!!

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