UA, DL, and AA all have credit cards that will get you into the lounges. I haven't looked at the details in a while, but the cards that come with lounge access usually all have an annual fee of about $450-500/year. You could also look at the Chase Sapphire Reserve, which gives you access to the Priority Pass network (check https://www.prioritypass.com/ for specific lounges at specific airports) with an annual of $450 or so, but they also give you a $300 statement credit per year for travel, so as long as you spend $300 on travel in a year it's really a $150 annual fee (plus another $100 credit if you apply for Global Entry). If you're only looking to get a card for a year while you're building status, the signup bonuses of 50000+ miles/points that all these cards come with will pay for the cost of the annual fee. You also can usually just buy a lounge membership directly from an airline for about the same cost of a credit card annual fee if you don't want to mess with a credit card, but then you don't get the credit card signup bonus.

At 1 million miles, I believe DL and AA give you lifetime of their lowest tier status, and UA gives you lifetime of their second tier status (of course, 10 years is plenty of time for United to "enhance" the program to match DL and AA).

For access to extra leg room Y+ seats, I believe all 3 will give access to "preferred seating" at booking with the lowest tier status - i.e., seats towards the front, but not necessarily with extra leg room and sometimes exit rows), then you get access to Y+ seats (DL Comfort+, UA Economy+, and AA Main Cabin Extra) 24 hrs before the flight. At 2nd tier status I think UA and AA let you book into Y+ for free at the time of booking, while DL doesn't let you do this until 3rd tier status (and there it's still an "upgrade" that needs to clear, not something you can do as you buy the ticket). DL also gives out free drinks in Y+, and I think AA is starting this soon if they don't already. In general I think UA and AA planes tend to have more Y+ seats than DL for most of the seating configurations.

One last thing to think about since you anticipate at least 80K miles per year (and possibly more with domestic and intra-India) are your chances of getting top-tier status. For AA and UA you need 100K miles (so not that far away) and $12K spend, but DL needs 125K and $15K spend. The big perk here since you'll be flying long-haul international flights is that you'd get upgrade certificates you could use for the international flights.