Which is the proper way (Just only) or (only just)?

I am trying to write something to my nephew, that I think will motivate him. In the message, there’s a part I can’t tell if it’s “just only” or “only just”. I’ve read through a few threads here, but the responses aren’t quite inline with what I’m asking.

It says,”I don’t want be a top producer, I just only/only just want to produce what’s on top.”

Out loud, I can stress where I want the focus, but it just doesn’t seem right when I read it.

What is the appropriate composition for this situation?

In this context only and just mean the same thing. Using both words conveys no extra information and is completely unnecessary. Native English speakers wouldn't use it. Use one word or the other. Since they mean the same it makes no difference which one you choose.

There are places where "only just" can be used, but this is not one of them.

I believe that the version of "just" that you are trying to convey here is the meaning where you are wanting to downplay or de-intensify the next statement, like in Marge Simpson's "I just think they're neat", or in the song line "I just called to say I love you".

If I'm understanding the humour you are trying to find here(ie. that the second statement is not actually humble at all), it may be appropriate in a personal note to replicate your out-loud delivery using an ellipsis ("…"):

I don’t want be a top producer, I just… only want to produce what’s on top.

You could also reword the second statement to move the only away from the just:

I don’t want be a top producer, I just want to produce only what’s on top.

I think if I was trying to write the same statement, I might change it further to something like:

I don’t want be a top producer, I just want everything I produce to be on top.