RosaA4780

My recommendation would be to either use a larger sized self tapping screw, or go one size up with a regular screw. You could. Go the JB route, but if it's going to hold up a fan I would rather one of the other two options.

Or replace the box. if you're comfortable shutting off the breaker and switching everything over.

I'd replace. The tank is compromised and leaking. Heatpump, conventional or tankless regardless of your decision, replacement will be needed.