"It's not Article 2 actually, it's the 22nd amendment which concerns allowing presidents to serve more than two terms"
You're right about that, but I was thinking more about the length of term. If you amend Article 2 then Trump doesn't need to worry about getting re-elected, he can just make himself president for however long he wants, President for Life even.
"The executive Power shall be vested in a President of the United States of America. He shall hold his Office during the Term of four Years, and, together with the Vice President, chosen for the same Term, be elected, as follows:"
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Then, to your point.
"...something Donald Trump has already mooted"
Yes, but in a very specific situation. He said "A massive fraud of this type allows for the termination..."
But my reply still stands to the initial question:
"How failsafe are those safeguards?"
They are very safe indeed.
In order to pass an amendment to the US Constitution then two thirds of both houses would have to vote for it (so quite a few Democrats would have to vote for it) and then 38 out of the 50 states would have to vote for it as well.
Just for context, only 30 out of the 50 states have voted for Trump this time round (assuming that Nevada and Arizona go Trump).
So, no, that is not going to happen.