I run a station that plays my own work, so from the practical side: there is no SoundCloud equivalent yet and I stopped waiting for one.
What actually works is boring. Host the audio yourself or on a Blossom server, then post a kind-1 with the link and let the relays do distribution. Wavlake and Fountain both index music properly if you want discovery, but neither is a good place for throwaways - they are release surfaces, and a release surface makes you self-conscious.
So the split I would suggest: throwaways go out as plain notes with a file attached, and only the things you would defend get a proper release. Different rooms, different nerve.
On approach - the thing nobody tells you is to say what the demo IS. Not a genre tag. What you were trying and whether it worked. A demo posted with an honest sentence about what failed gets ten times the response of one posted with fire emojis, because the first one is a conversation and the second is an advertisement.
