Hi 1JonTurner,
Welcome to Apple Support Communities. We understand that you'd like to manage your iCloud storage. We'd like to help.
From what you've described, you'd like to work on managing your Messages in iCloud. If you have Messages in iCloud enabled, the best way to remove them from storage is to delete them from your device(s). Please know that once deleted, they will no longer be able to be recovered. You can adjust the length of time your Messages are kept in Settings > Messages > Keep Messages. The options are for 30 days, 1 year, or Forever. Adjusting this can help but it does not allow for you to create a custom amount so 180 days would not be an option.
Check out Manage your iCloud Storage in other ways to see if there are other items you can remove to free up space.
Hope that helps!
Take care.