In the Help/Placeholder Tag List… menu command, search for “Restart” for help on resetting numbering streams. With compile settings like these, you could enter the reset placeholder into the chapter title suffix, since that is when you would want subdocument numbering to restart.
To use sub-chapters I see two options. If your document is like a novel and has multiple scenes per chapter then you could play with the template for a “novel with parts”. Treat each part as a chapter and the next level down as sub-chapters. You might be better off using something like the “hierarchal nonfiction” template and <$hn> to get the numbering that you want. With this, you would have to make everything you want to appear in the sub-chapter in the same document.