Electronics before 10k ft
 

I have an iPad Pro with the magic keyboard. Is the flight attendant gonna make me wait till 10,000 feet to start watching videos on it?

YMMV, but generally if it has an attached keyboard, they treat it like a laptop.

Which country, which airline?

I've never had an issue the a laptop as soon as we're in the climb phase (a couple of minutes after take off).

US on Southwest. I don’t have to use the keyboard, just hold it.

I usually start a show/movie as soon as I settle in and have the keyboard attached, only been asked once to detach it. Guess it depends on the FA. Just detach it when/if asked, no big deal.

Just detach the keyboard and you won't have a problem. You can even watch it during taxi and takeoff.

You should be fine. It seems like if it can be slid into the seatback they'll let you keep it out of your bag. I think I recall hearing there was a weight limit of 2lbs which I have to think your tablet is below. Detaching the keyboard would almost certainly ensure it's okay. I have a small Lenovo laptop that weighs about 2.5lbs that I rarely am asked to stow and it's got a permanent keyboard. I'll often just throw it in the seatback when they are "hawking" just before leaving the gate then pull it out just to be sure.

This is your best bet.

Large electronics such as laptops are prohibited during critical phases of flight (below 10,000 feet) when the need for an evacuation is most likely (as slim as it may be). A large device could create a trip hazard or otherwise impede egress. This is the same reason they are not permitted in seatback pockets. I don't have the specific example but most rules like this are "written in blood," which means it has happened before.

-J.