This is a couple of years old, but Aikar’s optimized startup flags for consistent garbage collection might be of some use. It’s a smaller set of JVM flags than you are using, and I’ve never seen some of those before.
In general, there are a few ways to optimize a Sponge server, including using and tweaking the entity-activation ranges, mob spawn caps, and/or tile entity activation, as well as the usual world pre-generation, and by the old reliable trick of lowering the server view distance.