Independent Consultant and Taxes

I am an independent consultant in IT working from home in Maharashtra with an income over 50 lakhs. Firstly, do I need any trade licenses like shop license (shop and establishment acr), PTEC, PTCR? Secondly, I did not raise invoice with GST for my clients till I was about to cross the threshold of 20 lakhs. Now if raise an invoice, I will be crossing the threshold of 20 lakhs (total would become 22 lakhs). I have started the gst registration process but it is not completed. The client is insisting on an invoice. If I raise the invoice without GST, will I have to pay the GST from my own pocket? In other words, does GST liability get triggered when crossing the threshold or when I raise another invoice after this? Thirdly, for Income Tax filing, do I wait till filing season and file my taxes under presumptive tax (Section 44ADA) i.e. opt in for this by filling out ITR4? Does it mean I do not have to pay any advance tax (since my TDS is being deducted by the client at 10%)? Thank you in advance.