Allocate customers based on their expenditure moving average

I would like to ask your support in a case I am working. I have a dataset around 80.000 lines from January to July where I have the customer expenditure and the moving average of expenditure based in 3 months. I need to check customer behaviour month by month to check if is increasing, decreasing or being stable and report in a new column in what group the customer is.

The problem here is that I have much oscillation between the periods which makes hard to classify the customer. I would like to have a measure to tell if the expenditure variation between the months is significative in a general way.

I have checked the moving average control chart, however it can give me only an ideia about who is consuming within a limit(customer with stable consume), however it is just a part of the problem.

I have also checked about some tests in time series cenario for stationary series.

Thank you !