Microservice throws RejectedExecutionException during load testing. What could be the issue here? - spring-boot

Question

I have developed a Spring Boot microservice which will be used very heavily (close to 10k hits/second). I have used Feign as my REST Client and used CompletableFutures to hit other services to get data in an async way.
In this one scenario, I am hitting another service for 4 different objects in one function using CompletableFutures.
My config is as follows :
Tomcat thread size :
server.tomcat.max-threads=250
Executor thread pool size :
private RejectedExecutionHandler rejectedExecutionHandler = new ThreadPoolExecutor.CallerRunsPolicy();
private ExecutorService executorService = new ThreadPoolExecutor(1000, 1000, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>(), rejectedExecutionHandler);
My code that I'm using to hit the external service :
CompletableFuture<Integer> future1 = CompletableFuture.supplyAsync(() ->
service1.function1(params) , executorService);
CompletableFuture<List<Object>> future2 = CompletableFuture.supplyAsync(() ->
service2.function2(params) , executorService);
CompletableFuture<Object> future3 = CompletableFuture.supplyAsync(() ->
service3.function3(params) , executorService);
CompletableFuture<Object> future4=CompletableFuture.supplyAsync(()->
service4.function4 (params) , executorService);
List<Object> response =Stream.of(future1, future2, future3, future4)
.parallel()
.map(CompletableFuture::join)
.collect(Collectors.toList());
These services are SpringBoot services which use Feign internally.
#Autowired
Service1(AppConfig config)
{
this.config=config;
this.currentService=connect(config);
}
#Bean
private Service1 connect(AppConfig config) {
Logger logger = new Logger.JavaLogger();
return Feign.builder()
.encoder(new GsonEncoder())
.decoder(new GsonDecoder())
.logger(logger)
.logLevel(feign.Logger.Level.FULL)
.target(Service1.class, config.getApiUrl());
}
On Load testing this Call using jMeter, which internally calls these 4 calls to the external service, I am able to achieve a rate of 200 hits/sec on 1 AWS instance. If I increase the load on 1 instance, I start getting RejectedExecutionException.
I tried the same with 10 AWS instances under a load balancer. Expectation was that it would scale linearly to 2000 hits/sec. However I could only achieve 1400 hits/sec. Any load increased after that caused the RejectedExecutionExceptions to come back.
What can be the solution here? Is there any tweak I can try here in the configurations?
Also, could Feign be the bottleneck over here? Should I try another client like Retrofit?