You might do better to ask this over on law.se, but here is more or less the answer that I would give there.
The "standard disclaimer"
This is a work of fiction. Names, characters, places, and incidents either are products of the author's imagination or are used fictitiously. Any resemblance to actual events or locales or persons, living or dead, is entirely coincidental.
has very little legal effect. If a plaintiff can demonstrate that the characters of a novel are intended to depict real people, or that some readers are likely to take the characters as representing real people, and that negative aspects of a character in a novel have been taken as reflecting on the plaintiff, and have harmed his or her reputation, then success in a libel action is quite possible, and the disclaimer will be of no help. After all it is a self-serving statement by the author. "I am not a thief" is not a defense to an accusation of theft.
And if the plaintiff cannot demonstrate all that, the absence of the disclaimer will not help the plaintiff's case.
At most the disclaimer establishes a lack of intent to describe real people. But intent is not a key element of a libel action. If a statement is false, and harms a person's reputation, it is potentially defamatory, even if made in the belief that it was accurate, or with no desire to harm.
The disclaimer does not hurt, and it helps establish good intent, but that is the most it can do.
To avoid risks of defamation suits, one might be careful that all statements about a character based on a real person are provably true. Or one might carefully make all characters based on a mix of multiple real people, plus fictional additions, so that no character is clearly identifiable with any real person.
Defamation actions are generally expensive. They are also risky, in that the original statements may be repeated many times in the course of reporting on the case, more clearly associated with a real person than the original novel ever was. (Consider the QB7 case in which Leon Uris was sued.) The chance of even a clearly defamed person bringing suit is not large, but it is not zero either. The risk will also depend on the jurisdiction likely to be involved. The US is notoriously less friendly to defamation plaintiffs than most European countries, for example.
The question says:
I would like to avoid being sued for libel, getting my work demonetized, a restraining order, or even getting a letter from a lawyer.
There is no way to be sure of any of that. Anyone may hire a lawyer to send a letter at any time, whether there is a valid claim or not. Indeed anyone may sue, even if there is no valid claim. Libel accusations do not normally result in restraining orders, or even injunctions (which are not the same thing).
An author in such a case might be wise to consult a lawyer experienced in defamation cases in the relevant jurisdiction. If the book is to be published by a major traditional publisher, the publisher would almost surely have a lawyer on staff or on retainer to asses such issues. Otherwise the author would have to make any arrangements. Initial consultations are often at low or even no cost, to asses if the lawyer is really needed and wants the case.
As a side note, I remember being amused by the standard disclaimer on one occasion. The novel Island in the Sea of Time includes the standard disclaimer. And on the first page there is a description of a character who is obviously based on a real person, another author well-known to Stirling, the author of Island. It happens that character is one of the heroes of the series, and there is no defamation. Indeed I understand that they are friends, and Stirling appeared as a bodyguard in a novel by the other author. But the "standard disclaimer" was obviously false in this case, and would have been of little value had the person depicted found something objectionable in the portrayal.