blueeyedviking I'm so sorry you've had all that to contend with, that is just really tough and horrible. You got through it, though, with a few scrapes, but you did and you're moving forward, that takes some guts.
I remember so clearly your boss being a complete nightmare a few threads ago and, as soon as you mentioned him here, I was all "oh what's he done now". I have a similar boss and a less than ideal work situation despite the job itself being a good one. In your situation, I would just trigger mat leave now and here's what I'd think about:
- you and bump must come first: you're in pain, that itself is exhausting and needs resting up/not making worse before birth, it is a drain for you physically and mentally, now is not the time to run on empty, whisper it, but there's a major life change coming;
- stress in later stages of pregnancy is agreed to be "a bad thing" for babies (studies vary on degree), and also a bad thing for you when you're already waking up at night with a bad back and pregnancy stuff and so on - not worth it for the sake of 4 weeks at work, especially if money is ok;
- impressions at work are already formed, people think you're good and on top of stuff etc (the way you wrote about that launch, they can't not, even I think that!) so you've got plenty of credit in the bank, 4 weeks makes no difference;
- you're having a baby, one of the most important and physically challenging things ever, everyone will understand an earlier departure both professionally and morally, you'll be letting go soon anyway (and I suspect if you did yourself proper damage the nightmare boss would just blame you anyway);
- work will still be there in 12 months time, or whenever you return, and either they'll largely be the same crowd or they'll have changed, but you will still have all of your history and silks to keep doing your thing, doing it even better - whereas you're only pregnant with this baby once and you can only get the start right once;
- I've not had a baby before but I gather being "ready" in terms of headspace, health and rest is a big part of getting on to an ok start, especially first time, and bonding, potentially even not getting PND in some cases (though I think in other cases unavoidable), so I'd invest the next 6-8 weeks forward into you/baby not backwards into a job that you've already earned your stripes in (even if you don't feel it).
An woman mentor type figure at work once said to me "always be where you are irreplaceable". That's what I'd do.
What I also would do though before you trigger the leave is not handovers but a fully updated CV and loads of notes for you on examples where you've been brilliant in this role - it'll give you perspective as you go, you're in no way going under a cloud, and will be jolly useful on the other side if you do have a bit of a confidence dip going back. Also I'd write an email to boss and HR placing it on the record that things have been far from ideal, list the stuff, say you're leaving early because of it, disappointed and (if you feel you can) that if you had more time you'd be minded to pursue more formally, but can they at least look into the problems so others don't have to go through this. I think with the sore back you wouldn't want a grievance process anyway, but I'd have it out there so it's beyond doubt what he got wrong. It's very unlikely they'd be stupid enough to insinuate anything (and what?!) but at least there's a contemporaneous record to protect you.
Sorry, big essay, lots of hugs and keep paddling!