A Python script may very well be easier - for what it’s worth, I also have an Automator script sitting around that takes two PDFs and appends the oldest-created one to the end of the newest one (the old PDF is not deleted). I use it with Hazel to attach the record of paying a bill to the original bill, mostly.
Evidently I can’t quickly upload it here because of the file format, but let me know if it would be useful to you and I can pop it up somewhere else and add a link.
(As well as “Combine PDF Pages” there is also a “Split PDF” action available in Automator which might be useful if you want to go that route. I haven’t experimented with it.)