Originally Posted by
Cpn_Dunsel
If there is actual flex that causes chain rub while under stress than the BB might have issues and should be inspected/replaced. It would not be the frame that is flexing but a bad BB can give a feel in the pedals that is concerning.
It could be frame flex that's causing the front derailleur rub, especially since this is an entry level bike.
Derailleur rub was a commonplace problem in the before time (before stiff aluminum and carbon frames became commonplace).