The elected representatives chose to delegate this power to the person they approved as the head of CMS. What principle would be involved in requiring Congress to execute its powers in a different manner (i.e. by not delegating), simply because one does not like the outcome in this case? Who would decide which powers Congress could delegate and which it had to exercise directly? Has someone already drafted the Amendment to the Constitution which would limit the powers of Congress in this way?

There's no reason in principle for Congress to directly make this decision about requirements placed on CMS contractors and not other aspects of vendor relations handled by CMS. (The narrow issue here is whether CMS can require its contractors to enforce vaccine mandates.)