See, if the details on the following documentation page work for you. Do write back if you need further information. https://experienceleague.adobe.com/en/docs/experience-manager-guides/using/install-guide/on-prem-ig/output-gen-config/config-native-pdf-publish/design-page-layout#multi-column-layout 

Hi Josh,For bookmap order of pages is governed by the bookmap itself. You will have to create a copyright dita topic and add in the bookmap. If you are using ditamap then what you are trying to do will work. Thanks,Vivek

Good to know Josh that it worked for you. Do post here if you have any other issues.

You will have to add your code in the footer div, then it will always be at the bottom of the page.<div data-region="footer"></div> 

You will have to redesign your back page a bit. position: absolute is the culprit. <div class="content" style="position: absolute; margin-top: 6pt; margin-bottom: 12pt; break-inside: avoid; bottom: 0"> 

Remove the Back Page entry. Go to the Page Layout order tab, add last_page layout as a custom page, and select the option merge before.  Generate the PDF and see what you get.

How are you specifying the last page in the template? Can you share the screenshot of the Page Layouts tab of your template?

Thanks, Josh. Do you need the customized numbering in the output or while editing? If it is for the output then let us know the output formats. Using CSS it can be achieved. Based on the need we shall share specific CSS code snippets.

Can you share the DITA source text?

Try in incognito mode of the browser. Just trying to rule out any caching issues.