I've flown my Phantom 3 Pro inside a factory (huge meat packing / abattoir) with lots of metal machinery and metal conveyors in the ceiling and metal reinforcements in the concrete flooring. I'll tell you about my experiences and challenges:
1. Initially my P3Pro complained that the compass was out of whack. So prepare for this to be an issue. You might have to calibrate away from the metal/machinery/metal reinforcements in the concrete floor. I managed to move it around on the floor and found a place where it would stop complaining and allow me to take off.
2. My P3 also complained about "electromagnetic interference" (a "red" warning so I assumed it would not take off). Yet again my solution was to move the Phantom a bit (inside the same room) in order to find a spot where it didn't complain. Thinking back, it would likely have been helpful to be able to raise it up from the floor - maybe just be the hight of a flight case or a small table.
3. I had VPS (Visual Positioning System) turned on when flying indoors. (I have it turned off for normal flights outside). It did give me the confidence that it would not initially drift when taking off - and it worked. But it only works to a certain altitude as you pointed out yourself.
I found that my P3Pro was very stable inside. The barometer still works (to control the altitude) and I did not have a problem flying above the VPS-altitude. But obviously it is flying in Attitude (ATTI) mode - and if you are used to flying only in GPS-mode where the P3 or P2 stops itself when letting go of the sticks, you should be aware that it will be more difficult to control. It will easily feel like it flies too fast indoors.
Flying indoors takes a lot of concentration and tiny stick-inputs. I'd suggest also changing GAINS/EXPO and increasing the length of the sticks (I write about these things on my website on the Tips & Tricks-page).
It might be useful to use Prop Guards for a little added safety. Touching something with the props will easily result in a crash - that will easily destroy the camera/gimbal.
I suggest you don't try flying very far away from yourself indoors because distance is very difficult to judge at distance. Just walk with the Phantom and take it nice and slow.
Good luck
It is doable - but it takes some care and time. Don't have a bunch of people standing breathing down your neck
I think you need to be a proficient Phantom pilot to try something like this. Personally I've flown the P2 hundreds of hours and the P3Pro 110 flights so far.
You might consider "hand-catching"/"hand-landing" where you grab the Phantom out of the sky rather than landing on the ground. This is not particularly for indoor-flying - I do this always, in order to not risk tilting or risking anything protruding form the ground hitting the camera when landing. Obviously great care should be taken as the propellers are dangerous.
Good luck!