There will be a gap in the navy until 2022 when you get those subs. Since you need a regional defensive approach against Indian navy and not a global strike naval force the best during that time is to go for indigeneous small hunter submarines. Some countries in the region for example Iran already has that capability. Small-midget subs also are less detectable if you acquire the technology to build them. Otherwise until 2022 you will be waiting with a gap in the navy.
Yes India has several A3 orion anti sub aircraft that needs to fly over your subs to detect them near your waters but you have plenty of bvr aircraft at your disposal to take out Indian 2-3 anti submarine aircraft flying over your waters. Also even if they are detected they are mobile and it is not that easy to hit them at least more difficult than attacking surface vessels that shine on the radar.
