If you want to use the API, you need an active Pro subscription. Any API features will stop working after your Pro subscription expires. However, just in case there's a terminology issue: you can make a custom character sheet without using any API features. When your subscription expires, you can continue using a custom character sheet. You just cant edit it any more. If you havent created any character sheets before, it's a bit ambitious to think you'll complete it in a month. Though if you do, here's my usual advice: make several copies of the campaign with the custom sheet before your subscription expires. That way, you can always start a new campaign using the same sheet, or have a backup if your campaign gets corrupted (rare, but does happen sometimes).
