I don't see why you're so down on the Argent Arts.
AM is our only perception and dispel Art, AS gives is the only thing that gives us a counter and AC is still useful when compared to Arts that are effectively a whole Realm above it. And that's of course, before all the passive dice they give us. And lets not forget all the Green level capstones that are waiting for us there...
AM is the 'good' argent art because it's so, so good narratively. Mechanically it's not that strong but considering so few use spiritual arts in the outer sect it doesn't really matter. We'll need to get a new perception art in the inner sect, as AM cap out in green and its element don't fit us, but it will have served us very well by that time we switch it.
AC and AS are brawler arts, one which focus on being in the frontline with a band of elite, and another which focus on being able to quickly get to such frontline as well as synergy with the way the offensive art force enemies to attack us. AC's part that is nice is the 'command ally to do more damage and get stronger yourself' tech as well as the passives, but even then half of those are not quite adapted to our style, and the rest of the techniques are designed for someone who wants to be constantly in melee. AS is the same.
If you compare it to, say, SEA, SEA's benefit from being attacked are not restricted to us being melee only, and most of our attacks from our arts are actually mid or long range if you count archery. AC and AS are both designed to work with someone who is built to be close range. Damage wise, AC is both poor and doesn't benefit from our better dice increase as well as force us to be in melee.
Then we get into the elemental preferences we have, and while neither AC or AS are
awful, as one is partly water while the other is partly wind, neither is optimal. Them stopping at early green also mean that by the time we will be able to get AS to green we will likely already be appraisal and working on finishing TRF and getting other appraisal arts, rendering it close to obsolete by the time we get it, considering the new cap on art passives.
So as far as the fourth argent art...
I want it, but FVM 7 is so important we should get it[A4] after tourney. If there's a "summer break" I think we should power through it then.
The reason I want it is that the argent arts appear to be set up as "Babby's first good art set" and so far we have Offense/ Teamwork, Perception/Spiritual Defense, and Phys def Speed.
It seems to me like the biggest "basic" block missing is spiritual offense.
Assuming you are correct in your premise (which I disagree with), the issue is that we could just get a spiritual offensive buff art that fits us by looking at the archive/better equivalent in the inner sect.
Beside Heaven/Earth just not being a good fit for us, we have been warned that while the Argent cultivation art is a good fit on top of familial cultivation art as they don't conflict, the actual combat arts are not advised unless you want to go all in and focus fully in the Argent Way.