"I knew too much."
Translation:Mi sciis tro multe.
Yes, you can. Tro can directly modify the verb. With adding "multe" it is clearer that you mean the amount, but not for example the deepness of your knowledge that is too much.
I think koni could work here, but it would be a bit awkward and the meaning would be different.
I know we're not learning German here, but this one explains the difference: https://yourdailygerman.wordpress.com/2014/11/26/what-is-the-difference-wissen-and-kennen/
koni – kennen
scii – wissen
I'm not sure if the meanings match 100% though, but I think it's a good approximation.
The reason I'm saying I'm not sure if they match is that Zamenhof also spoke Polish, and I saw examples which could use wissen in German but couldn't use wiedzieć in Polish – both of those verbs translate to scii.
But those are edge cases.
Tre translates to "very." So that would be saying "I know very much." I haven't gotten to superlatives in Esperanto yet, but imagine there are better ways to express how much knowledge one has than using tre. Tro works though.