Can anybody point me in the right direction on how to set up an App catalog for a single site in my tenant (SharePoint online)?
App Catalog can be setup at Site collection level and not site (web)/subsite level.
You can create a site collection app catalog using SharePoint Online PowerShell.
Add-SPOSiteCollectionAppCatalog -Site https://contoso.sharepoint.com/sites/marketing
By using PnP PowerShell:
Add-PnPSiteCollectionAppCatalog -site https://contoso.sharepoint.com/sites/marketing
Source: Use the site collection app catalog.