The documentation claims you can customize the executor service of ReactiveResilience4JCircuitBreakerFactory but that API does not exist on the Reactive version, only the non-reactive version.

https://github.com/spring-cloud/spring-cloud-circuitbreaker/blob/d715dbd63606b42cc56f645eeffe703d6328d6e9/docs/modules/ROOT/pages/spring-cloud-circuitbreaker-resilience4j/default-configuration.adoc#customizing-the-executorservice

reactive factory
https://github.com/spring-cloud/spring-cloud-circuitbreaker/blob/main/spring-cloud-circuitbreaker-resilience4j/src/main/java/org/springframework/cloud/circuitbreaker/resilience4j/ReactiveResilience4JCircuitBreakerFactory.java

non-reactive factory