Bag packing India

Hi all, I just finished my 12th and me + one of my oldest friends are seriously planning a 20-25 day low-budget backpacking trip across India (June) before college/life fully starts.

We’re both from India (I’m from Chennai, he’s from Pune)

and the rough idea right now is:
Chennai → Pune → Delhi/Punjab → Himalayas and then flight back home at the end.

The goal of the trip is intentionally:
Non AC sleeper trains
occasional hitchhiking
cheap hostels
gurudwaras/langar/seva
meeting random people
traveling light
Impromptu travelling based of strangers recommendations

The idea is to not let it become luxury tourism or “content creator travel.” We genuinely want to experience India properly while we’re still young enough to do dumb memorable stuff.

A few things I’d love advice on from people who’ve done similar trips:

1.Is a 40-50L backpack enough for ~25 days?

2.Best clothing setup for India backpacking? (I for the life of me can’t do those plasticky polyester gym clothes)

3.Laundry realistically while moving constantly?

4.Places in North India/Himachal that are worth slowing down for?

5.Sleeper train survival tips?

6.Smart ways to save money WITHOUT making the trip miserable?

7.Things you packed but never used / things you wish you had?

8.Any underrated places, routes, or experiences?
Anything two overconfident 18-year-olds are probably not thinking about right now

Would genuinely appreciate any advice/stories/tips from experienced travelers/backpackers.