Fabric

The world tick runs on one main thread, so per-core speed matters more than core count. More RAM only helps until the server has what it needs. Past that point it can cause longer garbage collection pauses. Real gains come from lowering simulation-distance and view-distance, pre-generating the world (for example with Chunky), and profiling lag with spark.

Players need the same Minecraft and Fabric loader version, plus every mod that runs on both sides, in matching versions. Client-only mods (Sodium, Iris, minimaps, shaders, etc.) must not be uploaded to the server: they are the most common reason a Fabric server crashes on startup. If you build your modpack in a launcher, remove those mods before uploading the mods folder.

On first start the server stops and asks you to accept the Minecraft EULA. Read it, set eula=true in eula.txt from the file manager, then start the server again. RCON uses the password and port from your Startup tab. The in-panel console connects to it. You don't need to touch server.properties.

Fabric docs