How does opening up external sharing affect existing and new site sharing settings?

Hi, you are right about the first thing. You will not "bump up" all existing as you're simply changing the org-wide default setting. New sites being created (without a group connected) have a default setting of "Only people in your organization" and when you create a new with a M365 group connected, which is required for a team, the default setting is "New and existing guests". Let's say you would have changed the org-wide setting to "Anyone" you'd still have to go a specific site to change that by clicking on the link in the site/group/team sharing settings to "Reset to organization-level settings".