WebSocket vs SSE vs long polling: decision framework for real-time features

we're looking at implementing real-time server-to-client notifications for around 100k concurrent users, and trying to decide between websockets, server-sent events (sse), and long polling. our use case is primarily one-way notifications, so the full duplex of websockets might be overkill, but browser support and corporate proxy compatibility are always concerns. sse seems promising for its simplicity and browser native support for reconnection, but i'm worried about connection limits on the server and proxy issues. long polling is robust but inefficient. given the mobile web context and the scale, what's a good decision framework for choosing between these? are there specific scenarios where one absolutely shines over the others, or pitfalls to avoid with each approach at 100k connections?