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The "living constitution" angle doesn't get you anywhere.Says who? The Constitution is in essence the basic operating manual for American national government. Conditions on the ground have changed — and we've learned more about the world — since 1787. It'd be unpragmatic (read: insane) to insist that the original modus operandi MUST stay the same forever and ever, just because that's how they did things in 1787, as long as the new M.O. can fairly be said not to be inconsistent with the constitutional text.
It doesn't help your case that Article V provides an amendment process: Nothing in the Constitution prohibits reinterpreting the existing language to accommodate new evidence and new insights, at least not as long as the reinterpretation doesn't do violence to the text.
"The executive power" in Article II is just as open to interpretation as anything. Interpreting that power as being subject to Congress's Article I authority is clearly well within the hash marks, to say nothing of the playing field as a whole.
Article I can fairly be read as allowing Congress to expand, and/or to cabin, presidential authority. That includes creating, and delegating power to, administrative agencies under general presidential supervision; that could include delegating expansive powers to agencies and perhaps limiting the president's ability to hire and fire agency personnel.
The "unitary executive" interpretation — with a president asserting the right to unilaterally disregard or revoke congressionally-enacted arrangements — is dangerous in the extreme. We've seen that in other countries, and we could well see the same thing happen here.
Decades ago, the head of the Reactor Department on my ship [the USS Enterprise] had a list on the office wall of Great Naval Quotes that we were forbidden to use. The #1 prohibited quote on that list was, "But we've always done it that way!" followed by #2, "But we've never done it that way!" Both of those are violated by using a bibliolatry-based approach to the Constitution as a purported justification for unilaterally up-ending longstanding practices — especially when, on the whole, those practices have worked passably well.
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> that just makes it more exigent to "pragmatically reinterpret" the law to ensure effective presidential control over the administrative state
That's clearly one of your ideological priors, but it wasn't handed down from Mount Sinai on stone tablets.