JCB has a partnership with AMEX so you will be fine. There are some Japanese articles about this
Announcement for user
Card usable: JCB usable = AMEX usable
Announcement for member stores
Since AMEX and JCB have a contract, AMEX is acceptable for almost JCB member stores
2 places you will visit accept VISA/MASTER/JCB so they will accept AMEX.
Universal Studio and Pokemon Center also have notes in English about credit cards
Credit cards AMEX・VISA ・Master ・DC ・JCB ・UnionPay
VISA, JCB, MasterCard and AEON Card credit cards and China UnionPay cash card are accepted at selected facilities within the park.