TL:DR: Just suppose that I am convinced to stay. In the many multiverses out there, what might this convincing conversation look like?
I'm in the highest position I can already attain in a company this small (head of all things admin + dealing with clients no-one else has time to deal with) on an average (for the position) salary, I'm integral to the company functioning (the only person I report to is the boss) and I love my colleagues and the work that I do, but I'm planning to put in my notice Monday morning as a result of the boss's pattern of behaviour in the past 3-6 months, after working here for over half of the company's operating life.
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I know it's highly subjective, but I was wondering if there were any cards they would have to play other than “Be insightful enough to apologise for their behaviour and not do it again”.
They're aware, as a standalone fact, that I am that integral. They've said things like “You know you'll always have a job with me” and I know they like how my work ethic has otherwise diverted work from them. The work I do will fall to both my colleagues and my subordinates, all of whom were reliant on me to fill knowlege gaps because they were all hired after me, and all of whom are working in a full time capacity as it is, so they're never going to want to fire me intentionally. But there is no unbiased HR in this company – that role used to be their wife, now it's just the boss.