I wouldn't say your question fits this site well, since it's more of tripadvisor type.
I'm familiar with all these countries, so I though to put my thoughts. There are many experienced travelers here than myself, so expect some better answers.
You can't "do" India in just 8 days, but it's certainly possible to visit many places and have plenty of Chais in 8 days (in Delhi). Agra is nearby, and I would spend at least two days for the Taj Mahal (including transportation and one day stay there).
Delhi alone is a huge city, with many large districts. You will probably get sick due to food and water there. 2 days would be enough to account for them. Delhi is way better than it used to be. Many people just overrate Delhi as unsafe or troublesome.
From Delhi, it would take almost two days to go to the other point in the golden triangle: Jaipur. Delhi and Mumbai (DEL and BOM) are the main airports, so any flights originating from other airports would cost you a lot more. I personally like Mumbai, and that's another day's train travel. A trip to India isn't complete without riding the train. If your departing flight is in the night, keep Uber or a taxi handy. Finding a taxi or public transport to the airport is difficult there.
If you plan to visit Thailand again, 8 days is more than enough for Bangkok. You wouldn't be able to go to Chiang Mai, or Somewhere north, but I don't see any reason to not try Pattaya or any southern regions.
Without breaking the budget, you can even go to Malaysia. AirAsiaX provides much cheap tickets to China, and most nationals have visa free or on-arrival entry
Two days for a river cruise + temples other side of the river. Other places are quick visits. There are many resources online about Bangkok. Stock up some clothing and food while you are at Bangkok.
I'm not going to write more about China since you have spent months there. 5 days wasn't really enough for me for Beijing.
Sunrise view from the park outside forbidden city is very hard to catch in this season. I would squeeze my Bangkok days and give more to Beijing.
Doing many countries in a single trip is definitely fun. You wouldn't be riding a train to different countries, but it feels badass to ask the lady in the next bunk which country you are in right now. (Try it on a Bangkok, Singapore, Kuala Lampur train!).
Make sure you have a backup credit card to rebook any flights if you miss any.
If you have never been to Bangkok or Delhi before, be prepared to deal with some scams. I'm not sure about your visiting dates. However, it's monsoon season in India, and visiting specially the southern places is very difficult these days. March and April would be perfect. There are holy festivals (seen in Bollywood movies?), and foreigners are welcome to put a load of of colored powder in strangers' faces!).
Visiting India without a solid plan is somewhat challenging. I'm from South Asia so I'm used to the customs there in India, but I personally know many who struggled to get around India.