Welcome to PhotoStructure, @Andreas !
Your library directory should be fine to run off a mergefs-type drive, as long as PS_FORCE_LOCAL_DB_REPLICA is true, and /ps/tmp is bind-mounted to a local standard disk.
In other words, /ps/tmp needs to support memory mapping/SHM for SQLite to work properly (which composite filesystems may not support properly). tmpfs is fine, too (and will be the fastest option).
Holler if you have any other issues!