So obviously the point of the experiment was for subjects to eventually open the door and have their spirit or mind go to some higher dimension. But what is the benefit for the researchers? This is apparently the 29th time they've conducted this experiment. They don't seem surprised to have many people open the door. This indicates that most or all people open the door in every trial. So they aren't still just seeing if it could be done. There's some other reason.
When Sakimura goes through the door, she says that the purpose of the experiment is to observe the new information in the higher dimension. But crucially, she says "we are the entrance". So I think they're still connected to their bodies, and the researchers are using that link to get information. After a few seconds of being in the higher dimension, I think it mostly destroys their mind. This is what the fireworks represent, a mind lit up and blazing, racing up further and further before exploding in a brilliant flash, then fading away.
The researchers interview the subjects afterwards to get what they can out of them, or maybe can get some information from some kind of advanced brain scanning technology. Or even using their brain/body's connection as a looking glass to get a glimpse of the higher dimension somehow.
And then they get rid of the subject by folding them up and through the door via the connection, maybe?
Although... We are assuming that the door leads to a higher dimension. There's something from Hama, I think, that talks about a dream she had as a child about the door. It may be something that is inside all our minds, and leads to knowledge locked away. Maybe for our own protection, by our own minds or some other entity.
In that case, getting rid of the subjects by folding them away is probably some gray tech that the researchers developed to dispose of used-up assets. Useful since it also removes them from the memory of everyone who knew them.