There is no need of a trial. It a was a qualification hearing. Does candidate satisfy all the requirements to run for office, including the test that he had taken an oath of office to defend etc the Constitution and then not participated in an insurrection against the Constitution. In this case and only in this state was the candidate questioned over the requirement of 13th Amendment Sec 3. Due process is accorded by the fact there was a hearing of determination and it was predetermined that which ever way the judge ruled, there would be an appeal to higher courts. That there was no trial does not abrogate actual due process. The judge made a finding i,e, Trump did participate in an insurrection, which carries enough weight throughout the due process to be pursued. Trump had an opportunity to testify but didn't. In MTG case Appeals Court dismissed the challenge. In other states an election board makes the determination.

There is plenty of due process.

When a candidate enters an election race their goal is to win i.e take the oath of office. If a candidate had previously taken the oath of office to defend etc the Constitution and then participated in an insurrection against the same, it would incumbent on election boards to disqualify such an individual from running. How that is done is different in the several states, but in each there is due process ti ensure democracy continues and remains Constitutional.