...you'll have to export GPX and import them into your master library.Note that GPX files do NOT support folders or lists. They are "flat". Thus when importing GPX file(s) one has to recreate any folders/lists in BC and manually move/copy data to them.
If I understand what you are trying to accomplish, I would suggest creating 10 or so databases using Preferences->Advanced->New Library. Once you are happy with the small libraries, you'll have to export GPX and import them into your master library.
According to my experience with BC, there is no easy way to merge two databases and I would really discourage of using the database format (*.gdb) at all. You can export to the gdb-format, but you cannot import without loosing all structure (folders, lists). That's almost useless for big databases. You would have to export every single list seperately. After re-importing all these lists to another DB you will need to manually rearrange your desired folder structure. That's a big mess! Unfortunately, there is no easy save and open option for databases included in BC.
I have tried to find a way on how to work with a BC DB that is stored in the cloud (OneDrive) and can be accessed by all my computers remotely and consecutively. The only way of working that seems to be feasible for me is to manually backup the DB after every single modification and to store that backup file back to the cloud. First step with a different computer is then to restore the backup file and after work is completed to backup again. Following this inconvenient procedure allows at least to keep the structure. Nevertheless, big DBs take some time for backup and restoring, unfortunately.