Looking for the best instant messaging software

Hi everyone, I’m planning to build a web/mobile instant messaging app with features like 1:1 and group chat, real-time messaging, offline sync, presence indicators, media sharing, push notifications, and scalable architecture. Has anyone here used MirrorFly for a production app? I’d love to hear your experience regarding scalability, customization, pricing, source code access, and overall reliability. Thanks in advance!