Can I go with higher CFM for bathroom ventilation fan?
You can certainly go with an 80 cfm unit, the only drawback being additional noise (in general the more powerful the unit the louder they are). This can be remedied by using a high-quality unit designed to be quieter.
You certainly can (and should IMO) use regular duct pipe in lieu of flex, wherever you can.
I don't think you need insulated duct for this. I live and work in a temperate region and am unfamiliar with considerations for vents in unconditioned spaces that could drop below freezing, but the fact that the existing vent is not insulated is a hint that it should be fine.