I also got the NotFound error yesterday. I tried again today and now it works. Are you able to set your connection string in the following line:
await client.Datasets.SetAllConnectionsAsync(workspaceCollectionName, workspaceId, datasetId, connectionParameters);
What you can also do is to use PowerBI-CLI, just to try if you can make it work. I received better error handling from it, which helped me to solve the issue as you read above.