I noticed one of our "local" brands also offers this frame, with ridiculous profit margins:
https://prorace.be/bikes/aero/randa, so I'm looking to order one directly from Yishun for a junior competitive cyclist.
I also had a chat with Yishun about their B2B/B2C. They are happy to sell directly to customers, however, not all models are available. The newer ones are usually reserved for high volume customers and only become available once the initial demand dies down.
The only downside for us is that they don't offer custom painting on single orders. You'd need to order at least two frames and two wheelsets to get custom painting.
W.r.t the headset: you may only need the top cap, and then fit your own spacers, stem, and handlebars. The bike linked above uses a non-Yishun handlebar for example.