I’d say for the lower saggy part of the bumper, get like an aluminium strip or something like that and bolt it to the inside of your bumper, since its down there in the dark you wont really see the screws or something
You should consider using carbon and applying your own epoxy to stiffen the front bumper. It's really easy to buy a sheet of carbon, glue it or something to the bumper and epoxy it
What is the brand of that aftermarket Aero bumper? I think I have the same bumper on my 93 Silvia front Vert! You can get the grille inserts from DorkiDori Rn for 145$ shipped! Also the license plate bolt spring nuts gives that bumper the lil OEM clean finishing touch!