Now which Constitutional power did Trump usurp by taking money allocated for a certain program and using it for another?
Probably none. However, Congress could have limited his ability to act in such a manner had they not deliberately chosen to provide flexibility to reallocate certain funds within certain constraints. Then he would have been violating their Constitutional control of the nation's purse.
Really? He had the "flexibility to reallocate certain funds within certain constraints?"
What were those constraints? What was the authorization that Congress had given him to reallocate the funds?
Certainly not 10 U.S. Code § 2808, which only allows him to reallocate military construction projects. So what is it?
If Trump allocated $500 billion to build a giant statue of Trump, he'd be violating the Constitution.
And what makes you think this isn't just a monument to Donald J. Trump? After all, it won't prevent illegal immigrants from entering the country. So what else is it good for?

There's no question that you'd flip your lid if he decided, for example, to declare that the DOJ would no longer be enforcing say laws against election interference. There is no legitimate discretion to enact DACA, through which the President unilaterally rewrote and violated the laws of the US.
I assume (hope?) that you'd flip your lid, too, if laws against election interference were ignored. I just hope we won't have to test that theory.

But, once again, he didn't "rewrite" laws. He decided not to fully enforce an unjust law. The laws are still on the books. Trump can enforce those laws whenever he pleases. (Or at least when the courts, the final arbiters of the laws, decide he can.

) So while you are quite certain Obama broke the law in enacting DACA, the courts have not agreed so far.
Not that every liberal is required to believe precisely what every other one is, but it strikes me as historically standard for liberal people to prefer peaceful international relations, while right-wing people believe more strongly in the strong military to create the peace. ... Can you really say you generally believe this proposition, or is it only in this case because it's Trump doing it for a project that you think is stupid?
If you really are for reducing the military's budget, shouldn't you be happy for this to happen for any reason at all?
To answer your second question, no, I am not happy and shouldn't be. Misusing his power to re-appropriate funds is a bad precedent, especially to use for a pet project that has very little benefit. I would also object if money allocated by Congress for military equipment was used to build a high-speed railway.
What amazes me is that no one seems to mind when Trump does it.
And, personally, I do believe a strong military is necessary to keep the peace. But it is not sufficient in and of itself to keep the peace. Diplomacy and soft-power are also necessary, and better used. But we need a strong military for last-ditch defense of our country.
It's when it's used as or considered to be the first response that I object to.