Import is not suitable for that purpose. There is unfortunately no way to bypass an extremely slow initial sync with file system sync on Android unless you have a rooted device and can manually replace the profile. I had a look before at implementing an alternate import feature to skip a long initial sync, but unfortunately because of the way the sync works it would be very complicated to get that to work, so I gave up.
It is by design that the import does not do duplicate handling and will generate new ids for all imported data. On the one hand, the definition of an import function is somewhat ambiguous and it is a common pattern in software that an import does not do any duplicate handling (whereas a restore function does handle duplicates, or replaces all data). On the under hand it is a common complaint about Joplin that this is not clear, and I agree there should be some kind of warning about this, because not knowing this can waste a lot of time due to how long it can take for Joplin to sync a large number of items