Congress is specifically granted its legislative powers in Article I. In section 1 the opening statement reads, "All legislative powers granted shall be vested in a Congress of the United States, which shall consist of a Senate and a House of Representatives." The President can't be said to fit that description. Congress is not granted the power to delegate legislative powers to another branch. That would override the designed separation of those powers.
If you're my age or older, which I think you are, all of this is 9th grade civics class material. The President is a few years older than I am and has a vastly superior education, yet he appears to have missed freshman civics in high school. If he opens his EO with the usual "by the powers vested in me by the Constitution and by act of Congress", then goes on to issue an order which usurps legislative powers of Congress, then he's either unread in the Constitution, or he knows better and is acting illegally anyway.
The 10th Amendment refers to the "United States" which is the name given by the framers to the federal government they were creating with the Constitution. That government is composed of three branches. There is no way to distinguish between any single branch and the federal government in regard to the limitations prescribed by the 10th Amendment. Those limitations apply equally to all three branches or they apply to none.
Consider what you just said. If Congress is to be able to delegate its legislative powers to other branches of the government, then they might hand over their power to legislate the institution of new taxes to the judiciary or to the President. They might be acting within their powers to delegate their power to determine the rules of Congressional proceedings to the President. That sounds like a sure prescription for disaster. Any President could seize dictatorial powers at will under your scenario, once Congress delegated any of the powers mentioned.
Each of the three branches of government are specifically empowered to perform certain duties. They are not granted blanket permission to do whatever they decide is expedient at any given time. You're arguing for the loose construction of the language of the document which is known as "the living Constitution". If we're going to accept that bizarre theory, we may as well have no Constitution.
If Congress was forbidden from delegating ANY authority, then federal laws could only be enforced by members of Congress. Bureaucrats would have no delegated authority to enforce Congressional laws...
Your misreading and misapplication of the Tenth Amendment would have members of Congress literally issuing federal pilots licences, rather than bureaucrats in the FAA issuing those licenses.
Your interpretation would deny mailmen the authority to verify that a postage stamp on any particular letter was valid. Instead, that letter would have to be physically examined by members of Congress.
That's simply not the case. Congress can delegate any of its power as it sees fit, so long as it has that power in the first place.