I can't tell you much about acting, because I see the shows subbed. However, the writing is terrible, in any language. Not only their choice don't make sense and there are constant plot holes, they are supposed to be seasoned Starfleet veterans, yet they act to dramatically, so emotionally, illogically and scared all the time.
And it's not a smart "logic vulcans vs illogic humans" confrontation, it's not a quirk of well defined characters with balance in their emotions; they are simply badly written for dramatic and sitting-on-the-edge type of narrative, which is fine I guess, but it's a cheap way to get traction in the narrative, which doesn't hold well for fans of the series that enjoyed the professionalism of Starfleet captains, supposedly the best of the best, capable of facing every menace.
Again, you want to do such a series, do it. But brand it as Star Trek, and you will get this kind of criticism.
The speed of sound is the velocity a force can propagate in a medium, so yes even assuming such a mega structure is theoretically possible (it's not) it would close its mouth at the speed of sound.
If you want a galaxy-doom monster, either pick a super massive black hole, a super massime white hole, a super massive black hole coming out of a bigger white hole, or a super massive quasar pulsating high energy X-ray bursts with enough power to kill all life of an entire galaxy