Auto Union function when exporting object

Hi all, Getting back to 3D design after rebuilding my Printer. I would love/need a Auto Union feature when I go to export STL or 3mf Files for 3D printing. I have ask for this feature quite awhile ago.
In the past I had to Union the Model to insure a water tight part. Doing this Manually in the past was very confusing to keep the Union and non Union parts/models separate because if I Union a part then I can not edit the individual part in said part/model anymore. If this sounds hard to follow in my post; Just imagine when I am trying to design something. SO, if we still have to Union a model before export can we please have an automatic Union feature of the model before exporting. Maybe I can export a folder of parts and the app will Union all parts in said folder and export as one part. I know that I am going to make iteration of the model so I need to keep a non Union version of the Model. Now, are you seeing the duplication complexity of my situation. Also maybe a unioned part could have a certain color so i know which parts are Union and which have not been Unioned. At least that would help somewhat. I would love to know any secrets to make this process easier. And how other deal with this issue.