1

It is a little cramped if you ask me. The most direct route according to google maps is 1709 km. But anyways, besides the places you've mentioned, Mahabalipuram is a good stopover between Chennai and Pondicherry. And for a hill station, Kodaikanal is a good choice.

If however you don't have any musts, I'd suggest a shorter route. If you go from Chennai to Goa pretty much direct, you can stop by, say Mysore and Hampi.

2

I'm writing this while sitting in Varkala, having spent the past 5 days flying in and out of Chennai to go to Pondicherry, and just previous to Varkala stopping in Kanyakumari--I think it'd be best to prioritize. Like ansh_jain_97 said, a direct route from Pondi to Goa offers some awesome waypoints. Otherwise, if Pondi is a negotiable stop (I enjoyed walking around and grabbing a stellar croissant, but ultimately wouldn't call it a highlight of my time in India), I would say you could start in Kerala and move up from there.

Varkala and Goa have completely different vibes, offerings, and environments, but at the end of the day, Kerala has many other sights than beaches, whereas Goa would likely leave you mostly beach-bound. So if you're looking for tons of beach time, go for Varkala and Goa; if you want a change of scenery, think about putting some more time in at the backwaters, hill stations/wildlife sanctuaries, or Kochi.

At the end of the day, most important is that you trim down your trip a bit. Traveling in India is not what makes this country enjoyable, so do your best to balance your desire to see many places with the reality that getting around is more difficult than many places. Happy travels!