What are the best options to repair this kind of damage in my bathtub?
In my personal experience, you can paint it any way you like and it will peel off again, sooner or later. The tub is not well made in the first place if it's separating like that, and this may be aggravated by being installed without proper support, so it flexes. But at more than 10 years old you're not going to get satisfaction from the manufacturer, if they even admit who they are.

That pretty much leaves the tub overlay (an expensive in the cash sense, yet cheap in the quality sense, even thinner plastic liner) and the unpleasant but effective (and generally cheaper if you DIY than the overlay) replacing the tub, with all the joy that implies for the surrounding tilework. If you go that way, at least pour a plaster support bed under any fiberglass/plastic tub, or give some serious thought to a cast iron tub - no fun to move but hard to damage once installed.