For PDF mergers, have you tried using Automator instead? I remember this did the trick for me when I had trouble with merging large PDFs.
Yes, in fact I came out with a quick and easy workflow to merge PDFs using Automator. Unfortunately, it seems this workflow would no longer work in future versions of macOS: My workflow for merging PDFs in Big Sur / Monterey
As a side note, Monterey broke down the video encode action in Automator (it can’t now convert videos into audio files).
I really wish Apple stopped the yearly release madness. They are breaking things at a great pace without ever slowing down to fix them.