You can use Outlook’s Auto Archive feature to automatically clear out and backup messages every few months or so.
https://support.microsoft.com/en-us/help/830119/description-of-the-autoarchive-feature-in-outlook
Alternate Article on Auto-Archive:
https://www.itsmarttricks.com/auto-manually-archive-emails-outlook/
When you do this, the only copy of the archived messages will be in Outlook’s archive folder on the PC running Outlook, so you will probably want to make a backup of your Outlook files periodically to an external thumb drive or hard drive with these instructions: