Welcome to the Airtable community!
If you switch a workspace to a Pro plan, that workspace will no longer be free, and you will need to pay for all 6 collaborators.
You could switch to a Plus plan instead of a Pro plan. You would still need to pay for all 6 collaborators, but Plus is less expensive than Pro.
If you do not need to be able to use an app, you could also see if someone has a legacy free workspace that they could transfer to you. A legacy free base can have an unlimited number of creators/editors.