Trying to distort the market so that lower priced items are more profitable is quite challenging to do without unintended consequences. A progressive consumption tax would definitely be a worthy experiment

When I say it should be free, it means that there is no cost to be paid by individual

No, I don’t support UBI, but I support UBS - Universal basic services. Food, housing, water, education, etc should be free at a basic level. Basic level for housing for example will be ‘Housing First’ concept in Finland.

I too would recommend that you start with something small that you want to build and google the best language for it and a guide.

₹719 for 1 GB/Day + Unlimited Calls + 100 SMS/Day valid for 72 Days. Post daily quota, data speed will be upto 64Kbps. About 3.4 eur per month.

Could totally do a sigmoid function and just integrate over the income. But the brackets are just discrete approximation of that.