I think that creating recurring jobs for each timezone based on a cron schedule of business hours in that timezone is a good way to segment your workload and keep your logic relatively simple.
From each timezone recurring job, I’d submit a Background job for each customer that has new data so that processing of the customer workloads is separated (and not synchronous) especially since you are sending emails.
One thing we’ve found is that our customers have different definitions of their “business hours” so you might want to keep that in mind. You might end up with:
Recurring job EST 9am-5pm
Recurring job EST 8am-5pm
Recurring job EST 12am-11:59pm…