Is there a reason you're looking specifically at elasticsearch? Just before pandemic hit I was starting up a load balanced ELK (ElasticSearch, Logstash, Kibana) stack for work.Thanks... what I really need now is an elasticsearch guru.
Wouldn't running an elastic load balancer, coupled with RDS, and cloud formation be a better option? Since xenforo is supported back end by MySQL isn't it? Cloud formation and ELB can be setup for auto scaling so when the site has little traffic you can scale back on hosting and when it gets busy auto scale up. Additionally, if you could auto scale it's possible that you can deploy updates with no to little down time.