Can I first export my entire collection and make playlists on the target USB?

Not true; the song is linked to the playlists, not copied twice.

So if I export one playlist with track A to the USb. And then export another playlist with track A and some other tracks, then track A will not be copied again?

Correct.

So as a basic management system, I should do the following:

1)Import entire collection to rekordbox and read files.

2)Export this collection to a USB drive A

3)Make playlists in Rekordbox and copy these to the drive A (provided the tracks in the playlist have been exported, only the playlist info will be copied)

4) to add new music repeat steps 1 and 2.

Assuming that I want both to search my collection and to make various playlists, is that the best way or is there a better way?

In terms of tagging, does it need to be done before the mass export? Or can I tag later my collection and somehow map that info onto the external drive A?

You don't need to export tracks prior to exporting playlists; if a song appears in a playlist, it will export it. If the song in that playlist is already on the device, it doesn't export it a second time.

You can modify the tags later then simply "re-export" the tracks / playlists and the metadata on the export device will be updated with the new information.