The smartphone advances of the last year have convinced my I should try to replace my:

US prepaid T-Mobile
UK prepaid Orange
France prepaid Bouyges
Brazil postpaid Vivo

and have one global plan. because I am often in Russia, Italy, Slovenia, Japan and China it would be ever so convenient to not carry a carryon group of sims plus the odd temporary local rental.

Does anybody have perspective on this? I feel inept and clueless despite a decent amount of research.

I really, really want an iPhone and am willing to buy an unlocked one, or even deal with an Android if I must.