The first part of what you said that should Trump have filed the suit directly on the federal process he would have to start at the bottom district court level to eventually reach the Court of Appeals and SC is correct
However this is a motion to intervene alleging he has legal standing and albeit this is a dispute between states, there is also a Federal question that directly impacted his reelection and thus he has standing to intervene This is a motion to intervene and if the SC grants the motion, it is confirming his legal standing and cannot later deny it.
Specifically, the brief reads,..”the violations of state election law, which is the “manner” the Legislatures of the States have established for choosing presidential electors, violates the Electors Clause of the U.S. Constitution and thus this matter arises under federal law. See Bush v Gore, 531 U.S. 98, 113 (2000) (“significant departure from the legislative scheme for appointing Presidential electors presents a federal constitutional question”) (Rehnquist, C.J., concurring).
Moreover, as explained more fully in the complaint filed by Texas, the number of ballots affected by illegal conduct of state elections officials greatly exceeds the current margin between Plaintiff in Intervention and his opponent in the election for the Office of President in each of the respective Defendant States, and the four Defendant States collectively have a sufficient number of electoral votes to affect the result of the vote in the Electoral College for the Office of President. Proposed Plaintiff in Intervention therefore clearly has a stake in the outcome of this litigation.”
First, this moves Trump's litigation to the front of the line at SCOTUS and short circuits his opponents attempts to run the clock out on him.
Second, having close to 20 states standing lends enormous support and credibility to President Trump's claims, boosts viability, creates political and public opinion. pressure on the Justices and add the talents and resources of 20 states AG’s and their staffs to his team
It also now provides a mechanism to get more of his cases before the court. As he files, it's very likely that the Justices may choose to consolidate the various cases in the interests of judicial economy.
Very nice move.