Translation of "this much and no more"
I think you are close.

Basically:

知道 = to know

我知道 = I know

我知道的 = What I know

就 = Just

這麼多 = this much

就這麼多了 = Just this much

I personally wouldn't translate this as "Beyond this I know nothing." It would be closer to "What I know is just this much", however I would translate it as "I only know this much". I don't see where this sentence would insinuate the "and no more" or the "beyond this"; it is not expressed in the sentence.

It's the type of sentence that would benefit from a bit more context to get a more accurate translation.