My modpack keeps crashing once 5 players join. Is it a ram issue?

Yes. You can see the Java heap space: out of memory errors, this is the modpack making use of the max amount, which inevitably crashes the server because it's too heavy. This is unfortunately the issue with a lot of modpacks on our service, we do not have the resources to support modpacks in fluent performance for all that long because of how resource heavy most modpacks are.