Microsoft project management software used to plan, manage, and communicate a project schedule and other information among workgroup members, project managers, and other stakeholders.
Dear Carlos,
The site storage should be considered. I guess that you use the Save site as template option to save a project site on the PWA site such as https://contoso.sharepoint.com/sites/PWAsitename/connected SharePoint site name as a template as the Save site as template feature may not support the parent PWA site.
If you want to back up the site content, you may notice the template size limit is 50 MB. If there are so many content in the site, reducing the site size may be needed.
Save, download, and upload a SharePoint site as a template | Microsoft Learn
The PWA site limit is 25 GB as you said which is mentioned in Project Online software boundaries and limits - ProjectOnline | Microsoft Learn. If you want to back up the PWA site, you may not use the Save site as template.
From the information "Do you recomend to reduce capacity usage exporting some Project´s *.mpp files and importing them into "MS-Project For The Web" (and deleting those exported Project from PWA)?", if you are using a Enterprise Project type, you can get all tasks in Power Automate and then import it to a new PWA site with a flow as a feasible way.
Project Online | Microsoft Power Automate
You can also open the tasks in the Project desktop client as a local backup. Then connect to another PWA site and publish the local tasks to it.
Connect to Project Online with the Project Online Desktop Client - ProjectOnline | Microsoft Learn
However, I recommend you use the Power Automate way at the mean time.
For some workflows created in SharePoint Designer 2013, you may operate the action in SharePoint Designer 2013.
If something is misunderstood, welcome to share any updates at your convenience.
Thank you for your effort and time.
Sincerely
Cliff | Microsoft Community Moderator