You’re overthinking this.
If you are a required crew member you can log SIC.
If your operation requires a 3-4 man Crew because of duty times or flight times you may count the entire flight duration as SIC.
Personally I don’t do that.
I just log in-seat time as SIC.
So your cruise relief pilot time is SIC time.
But you still need 1000 hrs SIC under FAA rules prior to upgrade.
That’s what the reg says.
Looking for 121.436 advice.