Should I delete or archive messages in my Sent folder?

There's no need to delete or archive messages in the Sent folder. Messages remain in this folder forever, unless you delete them. But, because you have 30 GB of storage space, you can keep messages in this folder to refer to them later, if needed. Also, note that messages in the Sent folder are actually archived in All Mail, so even if you archive these messages, they stay in the Sent folder.