Yea, honestly sanding is the only option you have. Nothing else. Literally, that's it.
But just so you know for the future, you should sand the paper mache before applying the gesso too. In fact, a way WAY easier way to go about this is to sand the paper mache, use PVA glue (the white stuff, you can buy a huge jug of it at a hardware store, it'll be worth it) and brush it on like paint, and it self-smoothes. that way you only have to sand the paper mache, then since the glue dries smooth, you can paint it however you like without it looking bad.
I did that exact method with this sword:
If you look, the blade is really smoothe. It's cardboard filled with expanding foam, and paper mached, then sanded and painted with that same glue. Then painted with the colored paints.