Currently we can turn on post-payments monthly convert for a given donation with the variant parameter
However as far as I can tell if a country is in $wgDonationInterfaceMonthlyConvertCountries, it's possible to switch to a different variant, but not to completely disable post-payments monthly convert.
Being able to turn this off via a URL parameter would be helpful for testing, and avoid us having to pester fr-tech for config changes (e.g. T291429). It could also help responsiveness if there's some urgent issue with monthly convert and no one from fr-tech is available.