Hi Ben,
Yes I would like to do a bulk update to everyone. I am using a webhook which successfully updates all subscribed/opt-in customers to "turn-on" for a specific subscription group in a sandbox test environment. The test environment had 1.2M contact which took nearly 4 hours to turn on 1 subscription group while our true list consist of 42M plus customers with 6 subscription group.
The rate limit for this is 5k per minute which will lead us over 40 days (at least) so this brings up a few hiccups. Updating 1 subscription group per run will take us at least 240 days to back-fill 6 subscription group for 42M plus and that runs into potential issue with new customer switch since they will run simultaneously and be part of the 5k threshold.
For that reason, I am hoping to switch all 6 subscription group to all subscribed/opt-in customers in one run then let the new subscriber webhook kick in after the run rather than reduce backfill and new customer switch. I tried testing multiple subscriber group switch which didn't work sadly. Hoping someone had better luck.