I know it may sound silly, but what is the difference between niz and ode? Could you use both here?
Also "ode" exists, if I am correct, literally only in two phrases: "ode mnie" (from me, than me) and "i zbaw nas ode złego" (and deliver us from evil, from "The Lord's Prayer").
Normally it's "od".
Niż is used with nominative and od/ode with genitive. Yes, you can use niż here, it would be niż ja.
I agree that "...than I" sounds too formal, but it is still correct, isn't it? I mean, when you are trying to learn a language as difficult as Polish, it is hard enough even without trying to guess what the program does or doesn't consider too formal.