Disruptive Frontline Gameplay

[Akimitsu]

Not only this, but a notable proportion of your team will be infrequent players who may not be so aware of what's happening and will be too late or even overlook Guard entirely.

This is the main issue, people being too pvp casual to even think of countering the premades, or worse, unwilling to even try to work with the rest of their alliance. Those people just deserve to lose, and they don't care, since they are only here for the XP. If people realized that -since a lot of them do the FL roulette at least once a day- they should take 10 minuts to figure out a burst and the rest of their skills at the Wolves Den Pier, things would go differently. This is also SE's fault, some tutorials might be good.

Reaper having such a short LB cooldown means a guaranteed Guard removal every single time

It does make it very bad to counter the DRK thing but let's not exaggerate, not every single time. I'll give you one thing though, no LB should work through/remove guard.

An advantage which quickly leads to defeatist attitudes amongst some within opposing teams which just exacerbates the issue.

I fail to see how the people who queue 1 time a day for their XP roulette could care, they get a reward whatever the outcome of the game is. For the people who queue multiple times a day solo, I'm not saying it's ideal, but with a little effort no premade is unbeatable. When I face one I usually try to lead a bit, or play jobs that can support the team to minimize the casualties.

through voice simply gives such premades an advantage that is poisoning the mode

They don't need to use voice, and most don't. They just pay attention to calls. Synchronizing things suprisingly only requires 2 things: being able to read and be willing to work with a shot caller.

Having played a lot of solo and premade both, I can also tell you premades usually aren't after a particular display of skill, this is FL after all, a mess. They're mostly interested in achievements or sometimes topping the weekly FL standing on Lodestone. Or just playing with friends.
And what makes them good, even above the DRK factor, is decision making and leading.