@ Indian2003 Welcome back to India.
Regarding the itinerary, it all depends on where all you wish to visit here.
Spiritual paths: Varanasi, Rishikesh, Kashi, Haridwar.
Nature tour: Kerala, Kashmir, Himachal, Leh-Ladakh, North-East.
History: Delhi, Agra, Ajanta Ellora, Khajuraho, Hampi, Jaipur & Udaipur.
Luxury Train rides: The Deccan Odyssey, The Golden Chariot, Palace on Wheels,
Normal train journey: Delhi-Thiruvananthapuram Rajdhani Express via Konkan railway, Jammu–Baramulla line, etc.
My take: Use any of the Luxury train rides.
They will take you to the most visited destinations.
You do not have to book 4* or 5* hotels.
You will be their exclusive guests so you can trust the food.
They have their own, tie-ups with luxury taxis/buses, tour guides, and hotels, so nothing much to worry about.
You can experience the journey as well as the destinations.
However, it is COSTLY.
As you are visiting after a long time, allow me to give you some advice:
1. Reduce the luggage as much as possible. Many of the things are cheap here and you can purchase, use, and discard them once you leave.
2. Have a cheap / mid-range phone (secondary phone with your bare minimum data and contacts ) and a good point & shoot camera. Do not use your iPhone -14-Pro-Max-Ultra in the streets.
3. Medicines: Have a copy of your medical records and visit a reputable hospital as soon as you land. Ask the doctor to refer to the records and prescribe the medicines which are equivalent to the ones you / your family are using there. Please do not go directly to any pharmacy to get substitute medicines.
4. Frankly speaking, I never recommend city sightseeing, unless you have some specific places to visit. Be it Mumbai, Delhi, or Bangalore, the traffic is horrible. Just check the Google map of any Indian metropolitan city traffic during daytime and you will understand.
5. Food: Avoid non-veg unless the eatery/restaurant is famous for its non-veg food.
6. Major tourist destinations in India will be crowded during vacations. While planning, make sure to give enough buffer time for every place you visit. As in India the IST is not Indian standard time, it’s Indian stretchable time.
7. As mentioned in earlier comments, digital payments work fine here. So please check if your Google-pay works here.
I wish you a very pleasant and happy time in India.