Hey Tan, looks like you've already picked up what you're after!
Attack magic potency and Thundara
I'd recommend Zircon rings over anything else outright.
The reason why attack magic potency isn't beneficial is because when casting Thundara after Thunder you gain 700 potency as a combo bonus. As a result potency doesn't benefit your Thundara which is why the player base throws a lot of INT on.
So anything that's giving you potency isn't really too useful in comparison to INT.
I recall a friend of ours linking me to some information which I have to keep annoymous incase he gets in trouble! They found that INT is roughly worth 3 Attack magic potency. That being said I don't know what spells they were casting so I personally don't put too much faith in that when it comes to Thundara.
I believe that the reason why Thundaga doesn't hit higher than Thundara is because Thundaga requires attack magic potency and the combo doesn't bolster it for Thundaga as it does Thundara. That however is speculation and I have no hard numbers to back it up.
Either way, I only find Thundaga worthwhile when you have Excruciate up to take advantage of its increased critical damage. If I'm short of MP I only cast Thunder followed by Thundara. I only cast Thunder when there're 5 seconds left on Thundara's cooldown to get the most MP back that I can during cooldowns from the recovery rate.
In regards to the weapon I used an Electrum Scepter +1 with INT melds to make up for the lack of accuracy on your rings.
Materia infomercial
If you're fed up with the % you keep getting on melds then consider a few things.
1 - A HQ item gains +5% chance of success
2 - Using lower stat materia first and higher stat materia afterwards boosts your success
(So, using a +16 and then a +17 will give you a higher % than a +17 then a +16)
3 - Don't be blinded by the tier IVs. Personally I see +16 as an average in regards to base stats. If I want to double meld for example my resulting stat needs to be 32 (16x2) but that doesn't mean that I have to use two 16s. Instead, try using a 15 and a 17 for a higher % of success. This can go even further if you choose to do so by having a 12 and a 20!
Now back to our regular programming
I should mention that this is what I used outside of Garuda, I always go Dragoon. Inside of Garuda you have got a accuracy buff up meaning that you can drop your magic accuracy for some more damage if you choose to do so. As mentioned in the previous post from Alatus, trading out the Mage's choker for the Spinel choker can be your last minute Garuda swap in!
I'm prepared for this post to get torn to shreds, going to look around for a source to the 700 attack magic potency increase on Thundara.
Black pearl ring & Critical hits
The critical hit rating from Black pearl rings and let's say... the Mogstaff (is that like the bowharp?) aren't really noticable due to Dlvl (the difference between your level and the mob level) which is a horrible system this game has in place to prevent you using fun stats. To be able to get any noticable difference out of these stats on higher level mobs you'll need to sacrifice your entire set-up which is not worthwhile as the increase is minimal.
I'm assuming Dlvl also influences how much damage your spells do on hit.
Pretty much the higher the mob level is the higher the critical floor is. To get your criticals to take effect more often and do more damage you need to go over this floor.
Let's say you're level 50 and you're fighting a level 50 mob. Your critical potency is 5 and the floor is 3. Your critical will do more damage than somebody with 3 critical potency.
Now let's say you're fighting a level 53 and the floor is 5. You will now do the same on a critical as somebody else with 3 potency.
Now I can't rememeber Garuda's level but the floor is pretty damned high meaning that any critical potency + and critical rating + you're picking up is not actually coming into affect whatsoever! It's a terrible shame and a really flawed system.
Note that in the same respect to a floor there's also a ceiling. If you were to critical a level 1 mob at level 50 you will have capped your critical potency meaning that stacking more potency will prevent it from going any higher.
You can find information on criticals right here!
<edit>
Can't seem to remembre where I found information regarding the 700 attack magic potency! Actual stats at least, anyway.
Here's something I did recall though!
Hmm... maybe I can convert this post into a new blog post...