Should I label AND archive a conversation?

Yes, labelling simply moves a conversation to a different inbox, it does not archive it.

You can label AND set a reminder or archive a message to mark it as done.

When the reminder is due or you get a new reply, the conversation will return to the label inbox.

Why would I want to do this?

Kondo is best used with an Inbox Zero workflow; and this applies to label inboxes too. Read more about Inbox Zero here.

What if I want to see all the conversations I labelled?

You can use the filter ‘all including archived’ in a label inbox. This will reveal everything you have archived and set reminders on in that label.