Anonymous...

I'm not gonna delve too deeply into the possible identity of the NYT's Anonymous "Resistance" Correspondent within the Trump Admin, I'm still not convinced the whole thing isn't an elaborate hoax, Trump himself would love to spoof the New York Times, that is if he had the brains for the job.
But...
Assuming the missive is legit, then some things ought to be discussed and detailed...

The letter itself has a whining exculpatory tone common to persons who fear imminent indictment, one is left with the impression that bad craziness is happening Behind the Scenes in the White House.
On the other hand, given the President's Primitive Paranoid & Immature Personality, why would anyone enrolled in a in-house "Resistance Cell" go blabbing to the New York Times when the short and intermediate term success of said undertaking depends on the maintenance of strict secrecy?
I think the writer's goal here, may be to provoke a public constitutional crisis, goad Trump into a berserker meltdown that cannot be walked back by his increasingly besieged enablers and thus either compel the Vice President & the Cabinet to invoke the 25th Amendment or else induce Trump to resign.
I doubt President Trump can be persuaded to resign save by a overwhelming threat he cannot deflect or evade and even then he'd want certain guarantees.  It is somewhat within his business profile to cut and run when the going gets tough, but he was usually able to "rip off a piece" on his way out, that is very much a potential situation that would require close scrutiny indeed.
As for the 25th Amendment, VP Mike Pence he was well selected in once sense, he seems blindly loyal to the President and exudes the baffled air of an imperiled contestant on the "Celebrity Apprentice".  Pence wouldn't bat an eye if Trump started mainlining thorazine in the middle of a press conference.
That leaves the time consuming option of impeachment which is still a political and logistical impossibility, for the moment...
My only other suspicion is that the author is likely not the VP or any member of the cabinet, it may well be someone fairly anonymous but with a great deal of access to the President and his in & out box, that suggests a staffer within the West Wing, other than that yer guess is as good as mine.