Monk:
Really Kabblist/Plauge doctor are your best bet there some will grab monk but i know very little about monk so i can’t help you much there just know its safe to know that monk is best if you want to be dealing nonstop dmg at max level as inquisitor.
The downside of this however is that we don’t know if future updates will close inquisitors dps gap making monk an unnecessary pick.
You will lose out on the support aspect from PD and Kabbalist at this rank just know neither would be considered mandatory.
Also if you choose to go Monk you should also consider Diev at level 3 i hear they work well together on some of their skill cool down/combos monk also makes many other Rank 3 classes viable.
Kabbalist:
Revenge Sevenfold, Ein Sof, Merkabah these are really the skills your going to use if you pick this class. Revenge sevenfold is easily underestimated its not an amazing skill but it can deal respectable dmg under the right conditions as it can block one shot on the whole team reflecting it for considerable damage. Honestly not worth picking the class for IMO just underestimated by some.
Ein Sof really is only there to keep you alive during rough parts of a fight with its +%HP. While the quick mana regen part of the skill is nice its not that useful as you can just pop potions when you need them. Its a great skill but ultimately overshadowed by other skills at the same teir.
Merkabah is interesting it used to deal incredible amounts of damage but now with the rank 8 skills its damage is paltry.
Merkabah was the main reason most players picked this class with a PvE lens on. Now Ein Sof is still quite good in PvP for the max HP boost but otherwise this class definitely seems to be seeing less use. Still one of my favs though.
Plauge Doctor
In this build Healing Factor, Incineration, and Bloodletting/fumigate are really the most useful skills.
Healing Factor is incredible as it almost instantly restores your health back to where it was when you first started taking damage. You can also link it to your whole group with C2 Linker from a Wizard making a two man farming wizard inq farm group insanely good.
Incineration is not that great of a skill by itself without Int but once you get inquisitor you will receive skills that effect its base damage before multipliers making it an effective skill.
Bloodletting and fumigate are amazing to have when you need them but you wont need them often.
Most players go this class with your current build it gives you more viability in PvP and quite a lot dps/support in comparison to the other classes.
Oracle:
The most useful spells are Counter Spell, Arcane Energy i can’t talk much on this class as i am not very knowledgeable about it.
From what i understand this class is only good for one thing and thats Counter Spell and its not really that good in PvE but its decent in PvP and supposedly that’s all you really need to know about the class.
Arcane Energy is also supposed to be nice but with only 15% extra SP at level 5 it seems to be kinda lack luster at lower levels especially with mana pots being so commonly used.
Pardoner.
A very over looked class. Most will only see it as a money making class which really isn’t the case. Its most commonly used skills are Simony, Indulgentia, Discerning Evil, Increase Magic Defense. The last being the most important to you.
Simony, indulgentia, discerning evil are all nice to have for both you and your allies. Simony gives you access to long buffs that you can cast far cheaper than from sellers, indulgentia helps clear status effects something PD certainly does better but this is a solid alternative. Discerning Evil can help keep your conviction up to help you have longer rotations with god smash/smite (hidden double damage modifier on god smash triple hit on smite).
Most teams struggle with magic type mobs at high levels of ET i hear never been there. Magic Defense is the most potent magic defense skill in the game outside of resist elements 19% chance to nullify magic outright.
So with access to both you can keep some level of physical def with priest Aspersion and Pally Barrier then alternate resist elements and magic defense to always keep at least 100+ magic resist on your team. some say its not worth it as the magic resist isn’t that important and that’s why you don’t see it very often. Plus as a DPS pally your not going to stack SPR which it needs.
The synergies are still there however and many like to play something a little more unique and more about buffing mitigation than just healing.
Conclusion.
Almost anything can be used at Rank 3 and Rank 7 almost anything works just keep in mind how they synergize together and more importantly that its something you want to play.
Also keep in mind rank 7 classes are rank 7 for a reason and their base modifiers/skills are just that much stronger than lower level classes.
Plague Doctor for an all around class that can do almost anything its more or less a standard pick.
Kabbalist for a fun class when using some of its less used skills some abilities have a greater focus on PvP but several skills such as 7-sided and Ein Sof can also help in PvE engagements.
Pardoner for something different and a great synergy with your other classes that you just really don’t see used enough.