* Make sure you run the latest 64-bit Calibre on the new machine.
* Create a new test library with one or two books on the old machine.
* Migrate the test library and check that every thing works correctly and you can access the test library.
* Make sure you use reasonable paths. With that many books I would think that shallow paths could be important. Make sure the Calibre library is located in a sub folder in the root folder of the HDD and not among any program folders.
* Migrate the main Calibre library like you did the test library. You can for instance restore a backup of the old library on the new computer, using an external HDD.
* Make sure you can access the library correctly on the new computer.
* Run "Check library" and fix any problems.
You can also simply remove the old HDD and attach it (possibly using an USB3 HDD dock) to the new PC and copy over the files. Keep the old HDD as backup.
Then consider splitting your library in several parts. For instance periodicals, fiction, nonfiction, garbage, new. Make sure that everything not in the garbage or new libraries is pristine without any duplicates and with perfect meta data and covers and is fully readable without any further fixes, conversions or editing.
Optional: Seek help for your hoarding disorder.
