I'm not familiar with ReportLab or pypyodbc, cx_Oracle...
since you mentioned mailing labels, I was wondering whether you checked out the new Report options that got introduced recently in ArcGIS Pro. This is something you can use on server as well.
That's too bad. Found a couple versions of this question on the forums, but no solutions and nothing I've tried so far works with ArcDesktop.
Don't have access to ArcPro for this project, but wonder how this scenario would be handled since Pro requires a conda env to install additional packages. Have to assume that it would honor the env during publishing right?