Reading the File Source from one site collection and provision to other Site Collection

Hi, 

We are in process of creating similar kind of PNPPartnerPack (As we have specific requirements we cannot use the same). Our requirement is keeping all the artifact files (like css, fonts, webpart pages, page layouts, images, js and pnp/xml template etc) of an template in document library (instead of Azure blob) of one Site collection (called SC1). in SC1, we are having a specific form to  request site collections using PNP templates. When a user creates a request to create a new site collection, we are extending the template XML during "PreProcessGetTemplate". We are ending up with an issue  "Value cannot be null" or "Path specified is not valid" in process of applying the template at the file (pnp:files) section source path level. Is there any best approach we can implement this requirement or how to fix the current issue?

Ex: <pnp:File src="{sourcesite}\Templates\Artifact\css\en-us\themable\corestyles\kp-theme.css" Folder="{site}/Style Library/css/en-us/themable/corestyles" Overwrite="true">