Budget for the prepaid expense as a fixed amount each month.

Add a Metric for the Prepayment Amount (use a separate metric for each prepaid expense for easy management)

Use sub-accounts on the Prepayments asset account to separate each expense and make them easy to reconcile and update

The formula on the sub-accounts is then The Prepayment metric - the Prepaid Expense account. This will calculate the monthly movement on that account.

In Cashflow Settings, Cashflow Forecast, Advanced, set the cashflow type for both the expense account and the asset account to Profile, Current 100%

In the Account Tree you use for cashflow purposes, create a Header for Prepaid Expenses and make it Summary Only. Add all the prepaid expense accounts and the Prepayments asset account to this header to combine then and see just the cash movement each month.