I could be wrong but I believe if we deepen our relations with Saudi, Iran will do the same vis-a-vis India.
Yes you are wrong. You dont seem to know or understand Iran. We dont care about Pakistan vs India and Iran does not see Pakistan as a rival. Iran's leadership is many things, but they are not juvenile enough to frame Iran's foreign policy towards South Asia through the petty one-upmanship that Pakistanis and Indians play against each other. Iran treats Pakistan and India equally and will not favour one over the other. So you will not see Iran trying to use India to spite Pakistan, Iran doesnt care about that. Iran will not allow itself to be used by India either. If you think otherwise then you just dont know Iran or understand the mindset of the regime (actually it is very clear from this website that Pakistanis on the whole are clueless about Iran).
Iran isnt opposed to close Saudi-Pakistan relations. Nobody in Iran cares about Pakistan to be honest and your country barely gets any notice or attention in Iranian political discourse. Iran only opposes what it perceives to be Saudi Arabian efforts to solicit Pakistani support for anti-Iran activities. Otherwise, Iran doesnt really take issue with Pakistan and isnt particularly interested in South Asia. It is Saudi Arabia that Iran takes issue with (and obviously vice-versa).
Back on topic.
To be fair, Pakistan's government usually resists Saudi pressure to engage in antagonistic acts against Iran. However, as the article opening this topic pointed out, Saudi Arabia has very close relations to different power circles within Pakistan that include both state and non-state actors. It is common knowledge that the Pakistani establishment doesnt function as a single unit, it's civilian government and army sometimes follow the same policies, but other times persue different, contradictory agendas. Saudia Arabia has close ties with both, and can leverage one over the other. In addition, Saudi Arabia has close ties to many autonomous Pakistani non-state actors which are influential in that country, particularly religious groups that run schools, mosques and other 'community' services that are independent from the Pakistani state.
Saudi Arabia without a doubt uses its influence over some of these Pakistani religious organisations to bypass the Pakistani government for initiating hostile activity towards Iran from Pakistani territory. Jeash al-Adl, like Jondallah before it, is very clearly under the wing of two Pakistani terrorist groups, the LeJ and the SSP / ASWJ. The ASWJ is not considered by Pakistan to be a terrorist organisation and a Google search will bring up many stories from the Pakistani media documenting its history of paranoia and hostile activities and rhetoric against Iran.
Pakistan has very weak civilian control over its Balouchestan where there is clearly a massive law and order problem. Pakistani Balouchestan is dirt poor, more than half the population is illiterate according to Pakistan's own statistics, its judicial system is weak and in some places, non-existent. Likwise, its police forces are weak and obviously unable to enforce Pakistani law and tackle crime. The schools and education sector likewise, are clearly in shambles.
So, in the absence of a capable police force, courts, schools and local government authority, Pakistani Balouchestan is unsurprisingly a haven for bands of criminals and terrorists.
As such, i expect that for many, many years to come, Iran is going to have to deal with insecure borders with Pakistan and cross-border terrorism. Saudi Arabia is just too influential among different Pakistani circles (state and non-state). Whenever Pakistan's civilian government refuses to formally engage in hostilities with Iran under Saudi pressure, Saudi Arabia always has the option of bypassing the Pakistani government through non-state actors and / or the Pakistani army and Pakistan's weak control over its lawless Balouchestan is the perfect haven for Saudi Arabian supported groups to attack Iran from.