NOSTRAS (https://app.nostras.app) has real NIP-F4 podcast support — publish/browse shows and episodes as native Nostr events (kind 54/10154), not a separate silo. It's genuinely young compared to Fountain (no discovery feed yet, just publish + play), so take that as an honest caveat, not a pitch — but if Nostr-native is specifically what you're after, it's there.