From my understanding of US politics, Congress / the legislative
branch has more power than the President / the executive branch.
Doesn't this mean that the midterm elections are far more important
than the presidential elections?
No. You really don't understand U.S. politics very well.
Presidential elections are also legislative elections
In Presidential election years, there are roughly the same number of legislative elections as there are in mid-term elections. Typically, there is an election for the U.S. House of Representatives and every seat of the lower house of every state legislature and approximately one-third of the members of the U.S. Senate and about half of the members of each state senate (except Nebraska, which is unicameral), every two years. (The schedule for state Governors and other state executive branch officials and local government officials varies considerably from state to state.)
A Presidential election has everything (on average) that a mid-term election does PLUS a Presidential election. This in and of itself, means that Presidential elections are always going to be more important than midterm elections.
The U.S. government has a very strong President
Your understanding the the relative strength of the Presidency and the Congress isn't accurate. If anything, the President is a bit more powerful than the Congress, collectively, in the U.S. system of government. The U.S. has one of the strongest Presidents of any democratic government that has such a post.
The U.S. President can veto Congressional legislation in a manner that is very rarely overturned, nominates executive and judicial brach officials whose nominations are approved ca. 95%+ of the time even when the opposition party controls of the U.S. Senate, the President can promulgate influential regulations and set federal government policies on myriad matters, and the President has almost complete control over how military affairs are managed and very broad discretion over foreign affairs. No treaty can be enacted without Presidential support while the President can reach executive agreements with other governments of considerable force. Congress rarely accepts all of a President's proposals or budget ideas (even when Congress is controlled by the same party) but both are nonetheless influential in policy making. The President has very little accountability to Congress. And, of course, the President has informal power as a center of media attention (called the "bully pulpit") and as leader of his political party.
Some of the "defects" of the U.S. Constitution involve the Framers erroneous belief that the executive branch would be weaker relative to Congress than it is in practice.
Presidential races are winner take all
All of the powers of the President are unified in a single elected official, while all powers of Congress are collective - no one legislative election determines its behavior.
Also, historically, Congressional races have not just been national fights for partisan control as they are in many countries with parliamentary government, because party discipline in Congress has historically been weak (partisanship is at a record high right now so this is less true now than it has been historically).
For example, in the highly controversial and partisan recent vote to confirm President Trump's nominee Judge Kavanaugh to the U.S. Supreme Court, both the U.S. Senate Republicans and the U.S. Senate Democrats have a defecting member.
The weakness of party discipline in the U.S. Congress is in part a function of the strong Presidency. Because the executive branch functions of government can continue without consistent support from a majority party in Congress that could be brought down with one loss of confidence vote in a parliamentary system, it isn't as critical institutionally for Congress to have strong party discipline.
Presidential races are usually competitive, legislative races often aren't
While most legislative elections aren't close (and hence aren't interesting) because most legislative districts and states have a clear partisan leaning, most Presidential elections are reasonably competitive or at least have the potential to be in many states, which draws interest nationally even in states that aren't competitive.
The President is more visible and easy for voters to monitor
Is the reason psychological (having a new perceived "face" of the
nation vs many new people in a vast organization)?
This psychological factor also plays a part, but not nearly as dominant a part as the question would suggest.
This is also a function of how the media interacts with the public. The voting public is very aware of the performance of the President whose activities are covered on a daily basis in the news and are relatively easy to assign blame to (indeed, the public tends to assign blame for the state of the nation to the President whether or not the President has had much of an impact on that or not).
But, discerning the effect on individual members of Congress or state legislators on particular outcomes is much more difficult to monitor. Only local media outlets (as opposed to national ones) cover this at all, in most cases, and even then, allocating responsibility is difficult. A legislator in a legislative minority party has little real personal impact on much of anything for good or ill, most of the time. Many members of the general public don't even know who their members of Congress are, but almost everyone knows who the President of the United States is.
In short, every Presidential election is a competitive winner take all event that also includes a full slate of legislative elections, while midterm elections don't have this character.
So, the premise of the question that the President or Presidential election years are not as important as midterm elections is mostly false.