Paris Gare du Nord to Gare Montparnasse, by public transport but without lots of walking?

The Montparnasse-Bienvenüe metro station is practically two stations with a long corridor between them: lines 4 and 12 to the north, and lines 6 and 13 to the south. The train station is on the south side, so whenever you're boarding a train it's best to arrive by line 6 or 13. (Here line 4 is still faster if you're an average walker with no luggage, and you won't incur any extra stairs, just extra corridors.)

From Gare du Nord, the best rail connection is RER line B southbound (take any train heading in the right direction). Change at Denfert-Rochereau for metro line 6 towards Étoile (the interchange corridor is near the middle of the platform on line B IIRC). You will have to go up and down a few staircases. Plan on 30–50min from TGV platform to TGV platform, not counting any queue to buy tickets.

If you want to avoid staircases altogether (apart from a few escalators to reach the trains at Montparnasse), you can take the bus. There's no direct line between Gare du Nord and Montparnasse, and there are many combinations with one change. Off-peak, plan on 50–70min from platform to platform, not counting any queue to buy tickets. At peak hours, the bus can get stuck in traffic for a very long time (less so between the second Sunday of July and the next-to-last Sunday of August). There is also not much room for luggage on the bus, so I don't recommend it at peak hours. An advantage of the bus is that you can go to Montparnasse 2 TGV instead of Gare Montparnasse; Gare Montparnasse get you close to the rear of the train, while Montparnasse 2 TGV gets you close to the front of the train. A good combination to have the option to go to Montparnasse 2 is line 38 (departs from Gare du Nord so you don't have to worry about direction), then change at Port-Royal for line 91 (towards Montparnasse).

A taxi will be more expensive than public transport, but the 30€ figure feels large. I'd expect about 15€ off-peak on a weekday (more at night or on week-ends; 30€ might be reached if you get stuck in traffic).

You can take the line 4 BUT exit directly and walk in the open air. It's not so long and doesn't have any stairs.

Take the RER B and change to the M6 line at Denfert-Rochereau. This connection is much faster then the M4.

Line M4 went fine for me as a direct connection between the Gare du Nord and the Gare Montparnasse. I did not have much luggage with me though, just a rucksak, a laptop and a small trunk. But enough space in the metro M4 to carry all that. Just take care that you take the M4 in the right direction. Ask people around you if you don't know, preferably in french. Some might answer in English. They are very helpsome. In 20 minutes I travelled from the platforms of the Gare du Nord to the platform in Montparnasse for the TGV to Rennes capital of Bretagne. I came from Brussels that day with TGV as well. I started at 10h00 in Brussels and was in Rennes at 14h30. Not bad, because there was a strike going on that day at the SNCF. But only 25 % of the Union members joined that strike, which is something I have never seen before in France. I was lucky.