Just in case those IPs are within your AWS account: you can apply a single security group to those machines and then use that security group as the destination in the outbound rule.
If they're outside your account then, you're right, that's a shortcoming in AWS (Azure and GCP both allow multiple destinations in a single rule).
Dumb question: if the IPs are coming from Route53 for web addresses, why don't you just point them as aliases to the same load balancer? Done and done, right?
If they're outside your account then, you're right, that's a shortcoming in AWS (Azure and GCP both allow multiple destinations in a single rule).