Hello Nikhil, I think North East part of India is the most amazing part of India. You can definitely cover Meghalya, Sikkim, Gangtok, Kaziranga, Arunchal Pradesh.
Buy a tripod, that really helps when you want to click all those great time lapse or your own pictures. Buy one with some good height so that your pics will look like someone clicked from a height...
Yes i slept many times in my car i had qualis and pajero few years back one of the best feature was rear seat and whole seating compartment converted into bed.
You can do it easily but wait for somedays because corona is not finished till now. trekking towards Kedarkanth is amazing and it can be done by non trekkers also.
Do some exercise, but good trekking shoes, take you thermals but only wear them during night, take good torches, hire a local to guide you for navigation, do try local food, try making new friends with locals...And dont use more phone, just observe and embrace the beautiful and magnificent mountains