What power has he gained.

He's apparently gained the power to arbitrarily shut down federal agencies, for one.

That's circular reasoning.

Joke's on you, circular reasoning works well when people accept it!

Explain how shutting down USAID due to documented fraudulent spending strengthens the president’s power. I can’t think of a single way myself, but maybe I’m overlooking something?

If he gets away with shutting down USAID by fiat then it makes it clear that he can go outside the bounds of law and demand anything of the executive branch.

The next step might be going to the DOJ and telling them to detain, let's say, Ilhan Omar on suspicion of treason. This illegal demand will have much more force because anyone refusing it will know that they stand on their own and have no recourse through the courts or congressional oversight.

Explain "documented fraudulent spending", who is making these claims, what evidence they've presented, and what groups have checked that evidence.

Off the top of my head and easily verifiable from their own website, how about funding "revolutions" in foreign countries? https://web.archive.org/web/20060130080730/http://www.usaid....