Hi Steve,
Of the two questions you mentioned:
- There are some features are not available in Office Online, like macro, but template files like document.dotx should be able to open in Word Online. For Excel files, the Excel template(xltx file) cannot open in Excel Online.
- About assign read-only status, for files upload to SharePoint or OneDrive, the read-only is permission assigned to to users instead of files. For example, an Office 365 group has a SharePoint group site, and the group members will have edit permission
to the site and the documents stored on the site. You could change their permission to read-only, then they will be able to download the files from the site and edit offline but cannot save any changes to the files online.
For more about Office 365 group: Learn about Office 365 Groups.
For more about permission levels: Understanding permission levels in SharePoint
The best practice to use document templates is set them as templates in the document library in SharePoint:

Click Add template to add the template you want to use in this document library. The Template 1 is a sample template I added to the library. When I create a new document, I could click New > Template 1, the Template 1 will open in Word Online and I can save
a document based on the Template 1.
But none of my changes will save to the template. Excel template has a different behavior since Excel template(xltx files) cannot open in Excel Online. If you upload a xltx file as a template, it will create a new xltx file in the library when you click
New and select the template.

You could use a xlsx file as a workaround since it will open in Excel Online and save a workbook based on it.
I hope I understand you right. If you have any concerns, welcome share more with me.
Regards,
Alex