In this setting, I have a species with incredibly weak vocalization abilities, being hardly able to speak over what a human would consider a whisper. In this, they have relied entirely on using their hands to speak their dominant languages. Some are assisted by other parts of the body or elements of body language, but hands maintain a mostly-universal method of communication within their cultures.
Since they are, at the time I am currently concerned with, at an FTL-level of technology, how complex might their sign language get after thousands of years of development and evolution? Writing could also be a subject of intense complication, as it has no need to actually be vocalized (human languages are literally constructed and organized around how we vocalize) and only translatable into physical sign (who needs syllables? or syntaxing? or even discernable letters?).
Some basic information that could possibly be relevant: This species is called the Shovan, an exonym given by their long-time brethren, the Scien. They live in wet and hot tropical climates primarily, being cold-blooded and thus unfit for the frozen worlds of the Scien. They have 5 digits on each of their two hands, as well as two legs and a clearly discernable head structure that features a large bony frill. Their scales feature intricate and colorful patterns, especially in their youth. They are on average moderately shorter than a human, but the digits on their hands are slightly (5-7 cm / 2-3 in) longer.
As always, all answers are appreciated and any needed clarification will be given ^^