Screen brightness for whatever you find comfortable. Only turn it down if you know you have a long day (or two) of work without any chance of charging. Ditto for keyboard. Optimized battery is good if you want to just use your Mac and not worry about the battery.
Don't try to micro manage - it is not worth the worry. There are probably benefits in keeping the battery between 40% and 80%
Yes to any of these. Easiest to just to plug it in and let Apple's optimized battery setting look after it.
I don't understand the question. Whilst power is being supplied, it is split between charging battery and running the Mac.
You may have noticed many answers have referred to
AlDente. This is the best app for those who worry about their battery. I use it.
When my MacBook is plugged in, I have this set to charge to 80%, at which point the power goes just to the Mac and the battery is neither charging nor discharging. In other words the battery is not being used - this must be better than manually messing about plugging the power in and out whilst watching charge level to keep it between 40 to 80%.
If I know I will be using without power for while, I tell AlDente to let it charge to 100% beforehand.
Is this worth the effort? Maybe, maybe not.
If you do want to improve(?) on what Apple supplies, get AlDente so that you can set and forget. I use the paid version.
Otherwise, do what most others do and just use the Mac. Let the Mac look after the battery.
Extra: The biggest killer for battery is temperature. If you live in a warm climate, watch out for high battery temperature. AlDente will alert you about this and attempt to keep it under control by reducing charge rate.