Donald Trump in a second term would likely install loyalists in key positions in the Pentagon, State Department and CIA whose primary allegiance would be to him, allowing him more freedom than in his first presidency to enact isolationist policies and whims.
I hate to say it, but until we can revamp our education system, educate the “low information voters”, and eventually vote properly with the idea of progressing our nation to help all, every election for the foreseeable future will be existential.
But yes, this one, I agree with you, is really the big one.