Trump’s no longer president, so it’s unconstitutional
First, the House impeached him while he was in office for something he did in office. The Constitution says the Senate tries after the House impeaches. It doesn’t say “except if the impeachable offenses happened towards the end of a president’s term and the Senate didn’t have time to try the impeachment until after he left.” There’s no period when the Constitution stops counting and the president becomes magically free from checks-and-balances.
Second, the Senate did have time, but Majority Leader Mitch McConnell decided against bringing them back from recess to conduct the trial.
Third, the Constitution explicitly states that impeachment is for barring convicted individuals from holding office in the future, not just for removing a sitting official. It makes no sense to say the Constitution is unconstitutional.
Fourth, the Constitution gives the Senate the power to determine the rules of the trial. Senator Rand Paul (R-KY) raised the out-of-office objection, and the Senate voted 55–45 that the trial can proceed. That alone is sufficient to make it Constitutional.