I want to launch 64 coroutines which will all be listening to a fan-out channel, I want the whole thing to suspend untill all 64 coroutines have been launched and are all listening to the channel, then I would return

You can use

launch(start = CoroutineStart.UNDISPATCHED)

and it will not return until the code in the coroutine suspends at the first suspension point.

the channel is rendevouz, and I start offering to it rather quickly, and the coroutines listening to such channel are still launching...

If you’re using offer with a rendezvous channel, you still have a race condition and could lose items even if all coroutines are started.