I am an expert GST practitioner. I advice many business about GSTR -1 and GSTR 3 both in less than 1.5 Cr and more than 1.5 Cr category. I can help you making templates to simplify returns in addition to filing monthly returns.
Also, I can advice you on generic GST related queries.
However, in your job, I am not 100% clear if you need help with coding or with calculation basis of GST. I can help you with both but would like to understand the ask in details.