I speak (and write) Chinese, Japanese, and a few other European and non-European languages.
I think, in the context of languages, saying a language is 'simple' or 'less simple' is not so straightforward.
For example, the Japanese writing system is fairly horrendous to learn, with a mixture of two different writing systems using syllables, plus Chinese characters thrown into the mix. However, it is also a reflection of how Japanese developed as a written language, illustrates past vocabulary borrowings from Chinese and gives a person who has learn it an ability to understand (to a certain extent) shared vocabulary as far afield as China, Korea and even Vietnam. I actually find Japanese written without the Chinese characters (as in books for very young children, for example) harder to understand, because you are left without a lot of informational input that the characters provide.
However, when thinking of complexity in writing, have you stopped to think that the same could be said of spelling in English? There are huge numbers of words with silent letters, as well as different pronunciations for the same letter combinations (think of how the following words are written versus how they are pronounced: cough, through, enough, though, thought, bough).
In addition to spelling, the grammar is some languages is far more complicated than in others. I started giving some examples here, but deleted them again, as it's just going into far too much detail. Put very simply, what I wanted to say is that if a language is 'simple' in one aspect, it often tends to be 'complex' in some other aspect, whether that is writing system, grammar, the range of sounds used in the language, and so on.
One problem with your hypothesis is that you are actually mixing two issues together. One issue is learning the fundamentals of a language, which happens in the early childhood years, before school age. The other is learning to read and write, which happens later. So the two things are happening at different periods of brain development.
In addition, brain development in young children is not just about stimulating the brain and having it develop more. It's more complicated than that. I read a book about brain development in young children a few years back, so don't remember too many details, but it did point out that in very early childhood the brain is developing all sorts of neural pathways, then a few years later it actually removes many of them - essentially aiming for efficiency rather than just bulk.