Yes, you do make a fair point about other galaxies having their own barriers similar to what our Star system in real life fas.
I still maintain the premise that the Mycelial network would have to exist inside the barrier but that its just not accessible to ships as a means of transport via the Spore Drive because of how the barrier affects ships... and that Disco (and other ships with a spore drive) can simple re-engage the Spore Drive once they move away 4-9 ly's at Warp (or other form of FTL that uses subspace) away from the galactic barrier.
Let's do ANOTHER Hypothetical: StarFleet & the UFP aren't under "Crisis Mode" and have time to do tests.
The Test Results are in:
They find out EVERY method of known FTL propulsion/travel through the Galactic Barrier is "Impossible" due to the damaging nature of the "Negative Energy Barrier".
You name it:
- Classical Warp Drive
- Borg-Style TransWarp Tunnels
- Voth-Style TransWarp Speeds
- Quantum SlipStream (Ver.1 & Ver.2)
- Spore Drive
- CoAxial Warp Drive
- SubSpace Vortex Generators
- Graviton Catapults
- GeoDesic Fold
- CareTaker Displacement Wave
- Soliton Waves
- Temporal Drive
- QUAHSSI-Driver <- My own Head Canon new FTL solution
Every single method fails horribly when sending in a automatic remote test vessel at FTL to cross the "Galactic Barrier".
Then what?
Just hop out into Normal Space and cross it like everybody else.
It's not a Big Deal.
Galactic Barriers are just Inter-Galactic Toll Boothes that you have to deal with by slowing down and finding your way through it.
Crossing the Galactic Barrier could even become a popular business venture once it's well understood.
You could see many private enterprises selling you services to take your vessel across.
Maybe make specially designed "Ferry StarShips" that are large & cavernous to fit your smaller StarShip inside and transport you across.
Its still 'convenient' that subspace seems to be perfectly fine and accessible to ships within a short impulse reach of the barrier, but the mycelial network is not.
Regular Warp Drive isn't Time/Energy efficient for covering the distances you need to travel from "Earth to the Edge of the Milky Way Galaxy".
I wouldn't be surprised if the Galactic Barrier has different affects for different forms of FTL.
But at the end of the day, you can use classical Warp Drive to get to the Door Step, which is fine.
Classical Warp Drive is a Good Enough of a method of FTL to cover the last few light-years.
At any rate, you also made a good point that the galactic barrier doesn't really present much of a problem for ships... but the writers did make it unnecessarily difficult to cross too in the 32nd century.
The Protagonists / Discovery crew are under a "Time Crunch" due to the DMA crisis.
So they slap together what they can to make it work instead of having the luxury of time to devise a more solid, fool proof plan to get through.
#1, Having only 1 layer of shields, seriously?
Given the rate of Shield Damage by the Galactic Barrier's Negative Energy, you only got 1 layer of shielding?
#2, Single Warp Core only? What if the Warp Core failed while you were traversing and you didn't make it to another Safe-Space bubble, you'd be up shit creek.
#3, You don't use Drones to help you create more layers of shield bubbles outside your bubble?
#4, Small Worm Holes obviously work for the DMA to transmit small amounts of Boronite Matter across vast distances, if you can fit small amounts of matter, you can create a small Micro-Worm Hole just outside of the Galactic Barrier using a smaller mini DMA like Worm Hole. Tarka was able to make one inside the lab of Discovery. I'm sure they can make one to cover the short distance that is the thickness of the Galactic Barrier & Maintain said Worm Holes, so they can transmit Subspace Radio messages via a giant Mesh Network of SubSpace Relay stations spread across the Spherical Galactic Barrier.
There was an entire plot around the Bajoran Wormhole and finding a way to transmit Subspace Radio signals through it. In the end, they only had to maintain a TINY, microscopic opening. That was enough to allow Subspace Radio waves to pass through to the other side.
Boom, you solved (Inter/Intra)-Galactic Subspace radio communications.
It ain't cheap or easy, it'll take alot of maintenance and ALOT of SubSpace Relays, but it'll get the job done.