Raining dead on the flag stand is what the mortar was made for and is not cheesy the slightest bit.
Let me explain the classic roles of Tribes. An organised team of tribes should have:
Heavy on Flag - A Heavy who stands on the flag and constantly looks around for incoming grabbers to shoot at them and bodyblock them. Normally should stay near the flagstand even if the flag is gone to guard it when it is returned.
Defence/Chasers - Sometimes different groups, often the same. Lights and mediums and sometimes heavys who defend the flag stand, hunt down enemy offense and chase enemy grabbers who hopefully have been slowed/damaged by the HoF. Chasers need to disc/thumper jump well to build up speed fast enough (the idea is, they regenerate while the grabber wont, so they can do that several times, Ascend maps are quite small for this though)
Offense (often HO for Heavy Offense) - Mostly Heavys typically whose sole job is to clear the flagstand for friendly grabbers to grab. Juggernauts and Raiders excell at this. Works WAY better with better communication as that allows to time the grab and clear. In public servers the way to do this would be something like saying VAF (Get the enemy flag) twice or something, but dont rely on your grabbers to understand at all.
Grabber - Lights mostly, sometimes mediums, who try to grab the flag. In a perfect world, they would get in position and say VTF (Target the enemy flag i am in position) like 20-30 seconds before they reach the flag to allow their friendly HO to do their job or even wait for their go. Often grabbers also say VFF (I have the flag) once they have begun their run to indicate to their team that nobody should llama grab (grab very slowly) the flag. Of course in public games that often doesnt work and expect to come in at 200km/h towards a cleared flag stand just to have one of your infiltrators or whatever walking over the flag.
Those are the fundamental parts every team should really have. Many people would add generator defenders and attackers, but that really is not as important as it maybe should be. One imho extremely important aspect of this for the techs out there: If you and both your turrets constantly are tied to defend the generator, it is not really worth it. Maybe if it is just you and some other tech repairs base turrets and sensor and helps defend flag stand, but i have seen way too many rounds where a single attacking juggernaut or infiltrator constantly ties 2 or 3 techs into the gener room and the gener does not help the greater good if all it powers is the gener defense.
This should only be a rough overview for people new to tribes. It is not complete or anything. Snipers, Infils and sometimes even heavy grabbers (grab of oportunity) have a place in the team. This should just hopefully give you a little hint what your team might lack (hint: it is not a 5th tech)
One last thing, because it drives vets crazy: Do not Llama grab. As in: Never walk while picking up the flag. You will get hunted down most likely and your actual grabbers wont have a chance to grab while the flag is in your hands. Only try a grab of opportunity if really no enemy is around and you are confident that you can build up enough speed fast enough to not be killed by respawners and nobody faster than you is visible.
OR when the enemy carrier is about to capture. That is called an E-Grab (Emergency) and there even is an accolade for it. That hopefully gives your friends enough time to retrieve your flag.
edit: I forgot another tip.
If your flag has been touched but the grabber died quickly and the flag now lies somewhere less open than on the flag stand (and in most maps that means anywhere near the base) do not instantly return the flag. Say VFS (Flag is secure) and stand next to it, looking around for enemys. If someone comes at you or someone like your HoF or flag carrier says VFR (Retrieve our flag) make a step ahead and retrieve it. In many situations that is WAY safer than retrieving immediatly, especially if the HoF died or left the flag stand himself.