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.

[Scintilla]

This is the main issue, people being too pvp casual

Taking time to learn the extent of their skills would help in a sense, not least by making some aware that Guard even exists, but by making them more aware of the full capabilities of their classes. True, it will allow you to focus more on your surroundings, more aware of defensive buffs/skills to help support your team in regular FLs (though this is not remotely enough to vs some premades). And, with practice, it can make you more aware of how these skills may be applied to better defeat classes regularly used by premades (use of MNK stun and knockback vs DRKs on Onsal podium etc). However, this not only requires a knowledge of your own class, but of other classes too. And the application of these skills in such a way will often only come with practice - just knowing the skill exists doesn't necessarily mean that a player will consider such applications. That will come more from practice than basic Wolves Den training.

But this still only applies to classic premades. Knowing your class means little when you are dragged into a DRK voke, instantly sent into Hysteria to cancel Guard and prevent actions, and then flattened by a spam of LBs. Where is the benefit in knowing your actions, when you can't actually use any of them

Standard RPR cooldown is 75s. Faster than most other class LBs, including DNC and WAR (two other classes who's LB removes Guard). Also, the use of Plentiful Harvest gives the RPR a buff to speed up the LB cooldown even further. With that used, it is more around a 60s cd.
By the time the LB has been used and the opposing team killed, the retreat, reheal, and the movement of the team to a new location to prepare another attack, it will be ready to use again.

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.

I feel it is precisely because they don't care that this defeatist attitude comes about: "get it over with", "just die to them and end this faster" etc.

Basically: No point trying, lets just get our reward and get out. It's an attitude that I hate to see. Premades can be beatable, but in the worst cases it often needs an unsaid mutual agreement between the two random teams to focus down the premade's alliance 2v1. Sometimes it happens, but not often.

My point is: this shouldn't be the case at all. If it comes to this, clearly premades are too abusable. I enjoy PvP, I don't want to see new players turned away from it because of this.

They don't need to use voice

This I cannot disagree with more. You get excellent coordinators in random teams who make a very strong team. But they never compare to good premades. If these teams are comparable to the premades you've seen, you can't have seen very good premades at all.