The way you mentioned is the only way that I'm aware of to do this, given that the settings in the portal are based on days and hours. MFA is included in the sign-in frequency settings, which you can control. https://learn.microsoft.com/en-us/azure/active-directory/authentication/concepts-azure-multi-factor-authentication-prompts-session-lifetime
I have shared your feedback with the product team for this feature and would be happy to include any details about your scenario, especially if you can provide a business justification. If you would like to raise a request yourself you can also do so on the Ideas forum. https://feedback.azure.com/d365community