I do get what you say, the problem is, that you both get to false conclusions, based on paper figures that are even wrong! As the Sukhoi website clearly states, it can't carry 12t weapons, but 8t, just as the Su 30s too. So there is no difference in carrying capability between the multi role flankers!
Also you look at the ferry range, without external loads in one direction and conclude that it could fly 1000s of Km into China, which simply is not realistic, because you completely ignore the capability of the opponent Air Force and their ground defences, let alone the fact, that the combat range which includes payloads and enough fuel to return, is the important one! Unless you have a stealth fighter like FGFA or UCAV, there is no way, we can send fighters let alone larger bombers deep into China and THAT's why we need longer ranger missiles to do the job, be it Agni' or simpler cruise missiles like Brahmos / Nirbhay (for MMRCA's that would e Scalp / Storm Shadow).
Brahmos launched from ground has limited range, but launched from an MKI at high altitude, adds several 100Km, even if launched from within Indian airspace and if the MKI cross over, it will remain in a safe distance to return home and let the missile do the rest.
You guys also look only at the wiki figures for payload, but not on the weaponload configs, to see which weapon can be used at which hardpoints. Su 30, 34 and 35 have all 3 hardpoints to carry KAB 1500 for example. So no matter, if they still have around 3t payload left on paper, you can't add more such heavy weapons, because of weight limits of the other hardpoints. So it's no so simple as you might think and you need to consider more things.
WRT terrain avoidance systems, as said before, there is no issue to add that into MKI too if IAF wants it, since it's just a matter of avionics and radar modes, the structural chages of the Su 34 have nothing to do with it. But with Rafale having these systems to and even excelling in that role, there hardly a need to worry about IAF on that regard. We even took the Israeli radar for LCA, because of it's good A2G modes, the same once we want to upgrade into the Jags, which also tells something about IAFs balanced approach on their multi role fighters.
Not really, they only use upgraded varients of the R73 or R77 that we currently have in our stock, but are on offer for us too. However most of the current upgrades are limited and the really new weapons are aimed on Pak Fa, which then will be passed to legacy fighters too. But they currently use the same R73/R77 A2A combo, with KAB bombs, Kh 29, 31, 35 and 59 too. In fact our MKIs already can use Israli LGBs and the Litening pods, with Brahmos and possible other Indian or Israeli weapons, the MKI strike pack is even improving beyond the one of the Su34 / 35 today.