It depends on who is intended to read or hear it. If it is some professional people, whose job it is to deal with these documents, then I wouldn't say it's too heavy. If you are trying to use it in everyday conversation, then it weighs 4 tons. I am a computer engineer. If I talk in technical language to other such people, it's not heavy. If I use it on friends who have no idea what I'm talking about, then it's much too heavy.

I would move the clause to the beginning of the sentence so that it reads: "Due to the limited lexical material available in these documents, this is not a particularly complex matter."

Quite simply, no, this is not too 'heavy'. Something is only 'heavy' when there are superfluous words (words you don't need), but each word in your sentence gives a specific meaning, and so it isn't 'heavy'.