Ok.
First, it's a conflict of interest to stay at your own hotel so you can make a profit. It's not an arms length transaction. Ethically sketchy.
Second, the Emoluments clause says the president is only allowed to earn his salary and nothing else from the federal government (and nothing from other governments). So he broke the law there. I know Republicans will respond that those charges were thrown out, and they were, but only because he had lost the election and the Supreme Court said therefore the point was moot.
Third, the most expensive hotel in the Trump chain (Central Park) charges less than $600 for a room today, and the Waldorf Astoria in Washington, which used to be a Trump hotel, charges $650, and we all know how much prices have risen the last few years. Imagine what it was in 2017. And Trump was charging over $1,100 per room. (Mar-a-lago is a private club, so "room rates" are a bit less obtainable.)
Fourth, I don't think you understand the federal per diem rate. It's not how much the government will pay to the hotel, it's how much the government will reimburse the employee, who is on the hook for the bill. But since those secret service agents couldn't afford to drop hundreds of dollars a night of their own money, they had to write excuses (lies?) on their travel forms to justify the extra costs and get 100% federal reimbursement (because the federal rate can be overridden for certain specific reasons, such as a lack of available hotel rooms in the area at the government rate).
