So since you're acting like unhappiness makes a horrible life for both parent and child, then I guess there aren't any good parents or pleasant lives?
Or maybe unhappiness is just an occasional part of life, and yes, in fact, people experiencing unhappiness can still make good parents.
Obviously people are going to be unhappy sometimes; I'll keep it simple.. I think a parent maintaining happiness is an under-rated aspect of what makes a good parent. Maintaining friendships, hobbies, making sure the parent's marriage is happy and getting divorced if need be, etc., etc.
That's it. I think "choosing the kids happiness over your own" is an oversimplification of how the parent/child dichotomy works, as in the long run your happiness might be more important than short term unhappiness the kid might feel. Obviously not in all situations, and obviously a parent can be more or less selfish.. just saying people ignore how important having happy and mentally healthy parents can be to the long term happiness of their children.
Sure, a parent needs to still have a healthy state of mind. They also need to at least learn how to hide that negativity for their kid's sake, and maybe even their spouse's sake. But if you can largely control yourself, It's almost certainly worse to go through with a divorce and shatter the one of the main pillars of your child's sense of stability. Avoiding divorce isn't about making your child unhappy, it's about providing stability in their formative years.
If your marriage has reached the point where it's direly toxic, then yeah, divorce is probably a better solution. If you're taking your frustrations out on your spouse of kids, verbally or physically, that's a huge problem that overrides any concerns about stability.
The problem as I see it is so many people are simply unwilling to work through difficulties in their marriage. Things go south and they split after a few months or maybe a year, rather than stick it out. Even if it takes years to get through it.
It's fine if your child sees you when you're angry or unhappy. It's OK even for an extended period of time, so long as you can still be there for them in the way you need to be as a parent.
On the subject of that trans parent, I would say he shouldn't have gone through with that even IF his kids have all reached adulthood. At the risk of putting my foot in my mouth, I don't see much difference between transitioning and having fucking Alzheimer's disease. In either case, you're watching one of the main sources of love and stability in your life wash away into something unrecognizable right before your eyes. Why would you ever willingly put your family through that, even if they're adults now?
Also, being 18 so very barely qualifies as being an adult. It's not like you turn 18 and a switch gets flipped. Christ. Your brain isn't even done developing until you're around 25.