Payment of Advance Tax for a Software consultant

Hello, I'm a professional software engineer who recently experienced a layoff from a multinational corporation based in Delhi. For the FY2023-24, I worked with this company for the initial 9 months, following which I transitioned into a role as a software consultant with a Turkish company. While taxes were duly paid to the government for the first 9 months of my employment, I understand that I'm obligated to settle the remainder as advance tax before the deadline of March 15, 2024. Consequently, I have several inquiries for the same: 1. Given my monthly income stands at approximately Rs 2 lakhs, and considering I haven't fulfilled my tax obligations for the final quarter of FY2023-24, I'm seeking guidance on how to calculate the tax for this period. Additionally, I'm interested in understanding whether I qualify for opting into section 44ADA for tax computation, considering my current status as an independent software consultant. 2. Is it mandatory to pay this calculated tax before March 15, 2024 as advance tax payment ? or can I settle it at a later stage while filing my Income Tax Return (ITR)? 3. I have also registered for GST and obtained the Letter of Undertaking (LUT) as well, but since my income for FY 2023-24 will fall below the 20 LPA threshold (as I worked as an independent software consultant for only 3 months in this year), so do I still need to file GST, possibly a Nil GST return? And if yes then could you advise on the appropriate GST form to be filled in this scenario? Thank you for your assistance.