I think the issue here is the 180 days period is not a fixed A to B. It is a rolling 180 days, so you need to wait for date A to pass at least one deposit before the running total will fall and create space for a further deposit.
I have never deposited cash with Monzo. Rarely do with any bank, fortunately, so I don’t know the text used in the feed for a cash deposit.
Is it as simple as “cash deposit”?
If so, can you search for that term, count back six months on the result, and tot up all the entries to see how much deposited in that time?
You would also see the date that first deposit would “drop off” the calculation.
Not ideal, I know, but maybe a solution for something Monzo will not amend.