I think it likely things will change before then.The 25th Amendment is designed for emergency replacement of POTUSes who are comatose or definitely right over the edge psychotic. It requires the sort of congressional majority that only the clearest of situation will generate and immediate action from the cabinet. It's not a practical solution for Trump.
Mueller aside, Trump looks like he is going to stumble through the next 3 years of office, fucking up, golfing, accomplishing nothing but the occasional cookie tossed at the 1% and hopefully not bombing North Korea. He's not really psychiatrically normal. It's evident to any casual observer that he may well suffer from Narcissistic PD, as he hits all the classic symptoms. He may well be slightly senile. And he appears to have no aptitude for the mentally demanding aspects of governing. If he was a medieval king, he would be later written about as Donald the Weak and the historians would note that he was ruled by his advisors and courtiers and that he screwed up and lost several border provinces to the neighbouring kings. He might even be killed and replaced by his nobles, as a result of his incompetence.
But in more civilized times, he is free to bugger around, rant, tweet, golf and watch TV until his term expires.
The issue is whether the GOP will replace him in 2020 or attempt to re run him at a time when the majority of the American people realize that he cannot deliver on his grandiose promises and will not provide any benefit to them.
Mueller's investigation is legit and there appears to be more then enough for charges on different levels, certainly on those closest to him.
Trump's tweeting will haunt him.
It'll also be interesting to see if the republicans ditch him. They are worried enough about 2018 losses, not to mention whether they can stick by him as his reputation becomes daily lower.
I doubt it would be the 25th, the republicans would really have to turn on him or really be worried about losses.