In the adventure bike space, you can take a look at the TVS Apache RTX 300, which will be the best all-rounder because of its form factor. If you want a neo-retro bike, you can consider the Honda CB350 models, Triumph Speed T4 and the TVS Ronin. They all are smooth, comfortable, and have reasonable performance, although the Triumph will have a noticeable edge over the other two. 

Of the three bikes you have shortlisted, we would suggest you choose between the Triumph Speed T4 and the Royal Enfield Scram 440. These two will be more comfortable, although the Royal Enfield Guerrilla will give you more performance on the highway. 

Since you've already owned a Royal Enfield Classic, the latest Royal Enfield Bullet / Classic is also worth taking a good look at because they will offer you good comfort and ride quality - both of which are important to you. 

You can alternatively also look at the Honda CB350 models because they are comfortable, smooth, well-built and torquey. 

A test ride of your shortlisted bikes will help you decide which is the best option for you. 

The Triumph Speed T4 is an excellent city machine, and you can also cruise on the highway at 100-110kph comfortably. It offers decent suspension comfort and will be a good fit for a one-bike garage. 

The Suzuki V-Strom SX will have more spacious ergonomics and better wind protection on the highway, but its smaller engine will also limit your comfortable cruising speed to 90-100kph. On the other hand, it will also deliver improved fuel economy in the city. 

Both options you have shortlisted are very capable inside the city and will be decent out on the highway, too. Between the two, we believe the Triumph Speed T4 offers you more for your money in terms of performance and badge value, too, if that's a consideration for you. The CB350RS is a good bike in its own right, and we would recommend a long test ride of both so you can decide which one you prefer more.

Between the three, we would recommend the Triumph Speed T4 as it is an accomplished city bike that can also handle highway cruising at 100-110kph. The Honda CB350 and Royal Enfield Meteor 350 are also heavier, slower bikes that won't be able to match the Triumph's handling on a twisty road. 

Considering your history of lower back issues, we suggest you go for the Hero Xpulse 210 as it offers a more plush ride quality. But do keep in mind that the seat height of the Xpulse 210 stands taller than the Triumph Speed T4 at 830mm, so you might not be able to flat-foot the ground. Another bike you can also consider is the Kawasaki KLX 230. Do take a test ride of all these bikes before making a purchase.