I have done stuff like this in the past by adding Public Groups such as "ArcGIS Pro Layouts and Styles" onto the Enterprise Portal and then adding Styles, Layouts and Project templates to them. They are then easily accessible to all Portal users in your Org, and can easily be added to individual projects. You can use Share as Web Style within Pro to add a style as Portal content. For Layouts and Project templates, as I recall I just uploaded the PAGX or APTX as content and shared it to the group.
As for making styles appear in everyones project by default .. I think you can in Enterprise portal Organisation settings (Map settings section) update the 2D and 3D Web Styles groups from "Esri Default", or configure the system styles as per this article .. How To: Configure Portal for ArcGIS with custom symbology (esri.com). I would never do either of these though because I would be too scared of breaking something!
As long as users know where to look in Portal in the Catalog pane they can easily find stuff ..
