How is it an overreach? It is actually one of the primary duties of the executive branch to defend the borders of the United States from foreign entities, Border Patrol agency is actually a recent thing, in the past the borders were patrolled by the US army and various law enforcement agencies. It isn't actually specified in the constitution that a separate federal agency needs to patrol the border, it's just generally more convenient.

This seems to be a response to the convoy of massive amounts of illegals on the way to the US, and due to the lack of increase in border protections from congress, this is the only option the president has. I'm not saying i approve of this use of resources long-term, but in the short term, activating some reserves or a national guard unit temporarily to assist on the border has not only been done before, but is perfectly within his power.