I'm very confused. Do you have a USA-type DL AmEx card or is there some special card for Japan residents? My experience form the USA side is that mention of the DL AmEx started to appear on my account and on boarding passes as soon as the card was approved, which was instantly on the phone. However, I did attach my DL FF account number to my credit card application from the beginning.

AFAIK no DL AmEx card gives you more DL program status, only indirectly when you earn MQMs on the Plat and Reserve cards from spend or from your initial/first purchase type bonuses. However, all DL AmEx Gold and higher cards do carry FO-type benefits (free bag, somewhat early boarding, etc., just not domestic upgrades).