For complete control, you can modify the source code and compile your own Eaglercraft client.
He explained: Eaglercraft mods are really just JavaScript plugins for the server. They don’t need Forge. They don’t need Fabric. They just need a server you control and a community that knows how to write web code. how to add mods on eaglercraft
You can’t drop .jar mods into a "mods" folder like Java Edition, but you can enhance Eaglercraft by downloading modified clients or using browser scripts. Stick to trusted sources to avoid malicious code. For complete control, you can modify the source