AWS NLB now supports multiple TLS certificates via SNI
aws.amazon.com
aws.amazon.com
At my company, we use Cloudflare's SSL for SaaS offering. It isn't exactly a load balancer per se, but it did allow us to have many users each with their own domains. We decided for our use case that it was better than rolling our own certificate management system for a couple thousand certs and figuring out how to hot load them onto web servers. https://developers.cloudflare.com/ssl/ssl-for-saas/