I haven't read the full opinion yet - and obviously I don't know enough about American constitutional law for my opinion about it to hold much weight, in any case - but at first sight, I agree it doesn't seem that unreasonable as a whole. Nor is it unreasonable, in se, for the Court to have wanted to take their time for such an important decision.

What IS unreasonable is that if Trump can run down the clock long enough on this as well as his other federal legal cases and then manages to win the presidency, he can just make the DoJ put an end to all of them (though, as I understand it, not the cases on the state level). Not even suspend them for the duration of his presidency, but end them once and for all. Whether he's guilty or not won't matter at all anymore, because he can just order the Attorney General to make them go away - and if for some reason the AG won't do that, take a page from Nixon's book and keep replacing the AG until he finds someone who'll do it.

And in that light, the Democratic fury about the decision makes a lot more sense. To some extent this is the usual clash between a conservative/originalist legal view that focuses on the principles of the Constitution without much regard for the de facto reality on the ground in today's America, versus a progressive legal view that's more interested in the real world outcomes and consequences. Just like all those cases where conservative justices reason along the lines of, Congress is free to legislate in this way or start the Constitutional Amendment process in that way, while completely ignoring the fact that Congress is far too dysfunctional and the country far too divided for any of that to ever happen in reality. Though in this case the theoretically correct but factually nonsensical assumption is more about how voters would surely punish a president at the ballot box for criminal actions and would surely not reelect him before he can be tried for them.

And then there is this beauty (quoting from the summary of the decision):

'The indictment alleges that as part of their conspiracy to overturn the legitimate results of the 2020 presidential election, Trump and his co-conspirators attempted to leverage the Justice Department’s power and authority to convince certain States to replace their legitimate electors with Trump’s fraudulent slates of electors. According to the indictment, Trump met with the Acting Attorney General and other senior Justice Department and White House officials to discuss investigating purported election fraud and sending a letter from the Department to those States regarding such fraud. The indictment further alleges that after the Acting Attorney General resisted Trump’s requests, Trump repeatedly threatened to replace him.
The Government does not dispute that the indictment’s allegations regarding the Justice Department involve Trump’s use of official power. The allegations in fact plainly implicate Trump’s “conclusive and preclusive” authority. The Executive Branch has “exclusive authority and absolute discretion” to decide which crimes to investigate and prosecute, including with respect to allegations of election crime. And the President’s “management of the Executive Branch” requires him to have “unrestricted power to remove the most important of his subordinates” —such as the Attorney General— “in their most important duties.”
The indictment’s allegations that the requested investigations were shams or proposed for an improper purpose do not divest the President of exclusive authority over the investigative and prosecutorial functions of the Justice Department and its officials. Because the President cannot be prosecuted for conduct within his exclusive constitutional authority, Trump is absolutely immune from prosecution for the alleged conduct involving his discussions with Justice Department officials.'

In other words: if you're a sitting president and you lost the election, you'd have to be a naive fool to respect that result, when the Supreme Court explicitly said that you're welcome to have the DoJ intimidate and manipulate states into falsifying the results as much as you like. You have nothing to lose - not unless there's a sufficiently large hostile majority in Congress, which in this day and age is unlikely, but even then, the worst thing that can happen is you get impeached and sail off into the sunset.

Of course, if state office holders stand tall and withstand all such pressure, as Kemp and Raffensperger did, the president or the AG can't actually force them to cheat - but there are very, very few Republicans out there like Kemp, who have both the resolve to take that stance and the ability to keep their career on track afterwards.

The decision doesn't actually go quite so far as that - for 'unofficial acts' it clearly says he's not immune, while even for official ones, it's not entirely one hundred percent ruling it out, just nearly. Again from the summary at the top of the decision:

'the Court concludes that the separation of powers principles explicated in the Court’s precedent necessitate at least a presumptive immunity from criminal prosecution for a President’s acts within the outer perimeter of his official responsibility. Such an immunity is required to safeguard the independence and effective functioning of the Executive Branch, and to enable the President to carry out his constitutional duties without undue caution. At a minimum, the President must be immune from prosecution for an official act unless the Government can show that applying a criminal prohibition to that act would pose no “dangers of intrusion on the authority and functions of the Executive Branch.”'

If Trump does win reelection, yes, yes it is. If in fact he doesn't, then the trial against him can still continue and he could still be found guilty, so indeed in that case it's not nearly as big a win as he thinks. But it's a big problem that justice is conditional on the outcome of an election like that.

Edit for formatting the quotes and adding the link.