Should I replace or fix my cracked fibergl tub?

Given all the issues with the current tub, I'd go for replacing with a steel tub. As you say, they don't cost much, and they are sturdy. They're also less fussy when it comes to cleaning. If the support under the current tub is as bad as you suspect, I think the current tub will continue to have issues. Repairing a fibergl tub from the top is going to be very time consuming if you want to get it looking even close to finished, and if it continues to flex due to poor support then the patch could let go eventually. I think tearing out what you have, fixing what's underneath it(if it's been leaking, there may be rot) and installing a new steel tub is the long-term fix. Maybe a little more trouble, but if it's done right you should never have issues with it.