Tds on remaining amount

TDS  rate on remaining amount should be @ 10% as the amount is payable only in the current fin.year, for which there is no such concession has been given by the Govt.

Yes, you deducted TDS as per Covid-19, relaxation provision. Which allowed TDS deduction at 75% of the actual TDS applicable over the payment.

For Advance payment made in last FY, the TDS obligation happened last year itself. Hence , it may not subject to any TDS in the current FY. Therefore, you need not pay any differential TDS on advance payment in CY.