Your argument is not sustainable for the following reasons:
In the instant case, value of supply received exceeded Rs.5000/- in a day.. It cannot be spread over the month.. RCM liability in this case is required to be determined in terms of Section 13(3) of the CGST Act'17 i.e. date of payment, which determines the tax liability of the recipient. Accordingly, the amount paid is the criteria, which in a day exceeds Rs.5000/- and hence RCM u/s 9(4) would apply since exemption under Nofn.8/2017-CT is not eligible.
Sn.13(3) - In case of supplies in respect of which tax is paid or liable to be paid on reverse charge basis, the time of supply shall be the earlier of the following dates, namely:––
(a) the date of payment as entered in the books of account of the recipient or the date on which the payment is debited in his bank account, whichever is earlier; or
(b) the date immediately following sixty days from the date of issue of invoice or any other document, by whatever name called, in lieu thereof by the supplier:
Provided that where it is not possible to determine the time of supply under clause (a) or clause (b), the time of supply shall be the date of entry in the books of account of the recipient of supply: