Illogical

Yes it is a bad movie

Normally if a Sci do movie is not action packed then it must be trying to give a message , but that's not the case here, no explanation for all the weird things that happened

While I had a few issues with some things near the end, I liked the movie quite a bit. A very well done HP Lovecraft style story.
I think more than half of your complaints were addressed in the film. Maybe not though ... it has been awhile since I've seen it.

As others have mentioned in other threads, the answers to those questions are all dealt with far more comprehensively in the book, but I think were all implied in the movie.

Ventress tells Lena that they have sent people, animals and drones into the Shimmer, and that the previous teams were primarily miltary. Nothing came back. She also makes the point that she picks the teams. The shimmer is getting bigger and will soon envelop occupied areas.

Basically, they are clutching at straws with mixing up teams and seeing what sticks. All military didn't work. Presumably all male or mixed teams didn't work, so they are sending in an all female civilian team.

In the novel, they have been sending in teams for thirty years, and some did come back, albeit in a bad way. From their reports they know that Area X has unpredictable results on technology, causing most mechanical tech to fail, so they send teams in with only really old tech. They can't send vehicles in. because there is only a narrow 'door' through which they can enter. I've no idea why they changed that in the movie, 'cos it clears up a lot of problems.

The makeup of the team is for very precise reasons, but the pyschologist has much more 'villainous' reasons for the selection, which doesn't come out until the later books.

I liked the movie a lot as an 'alternate universe' version of the book story. They could have made it clearer, but as I said, the implications are there.

"...into this zone they have no idea what is going on."

Ventress actually knew quite a lot about the Shimmer including it's higher dimension. This adventure for "scientists only" has only one mission- to reach the lighthouse and discover what is causing it. Later when Anya argues that "We have to go back now...We have data, observations, photographs. She has hella footage.", Ventress's response is "All of which makes the phenomenon less explicable, not more."

"Why not send a few soldiers with them?"

Ventress knew at some point team members might turn against each other. In such case, female scientists won't stand a chance against male soldiers.

"Why not go by helicopter ... jeeps or ATVs?"

Electronic devices do not work properly in the Shimmer.

"The rest of the movie was just slow and strange"

They are scientists on a research mission, not a strike force on the decapitation mission. Therefore, the audiences need patience.

It's an acid movie. Watching it straight won't really get you anywhere. Watch it fucked up. It takes on a whole new life of itself.