Retrieve Term Navigation Properties using REST API

I have faced a similar issue. The SharePoint REST API v2.1 often does not expose navigation properties like simpleLinkUrl or targetUrl because the "properties" field usually returns null unless custom properties are explicitly set. Navigation URLs configured in the term store might not be accessible through this endpoint. Also, $expand does not always work as expected. While you prefer not to use Graph or PnPjs, the Microsoft Graph API currently provides more complete access to taxonomy metadata including custom properties on terms. If REST API is not returning the data you need, using Graph might be the best option. Hope this helps.