Since your priorities are comfort, mileage and spacious ergonomics, here are some options you can consider. Given your long-standing relationship with the Thunderbird, something similar to it from the Royal Enfield stable would be the Meteor 350, which is a genuinely usable cruiser on our roads. However, its siblings - the Bullet 350 and Classic 350 - are more comfortable due to their upright riding positions, and those would be our recommendation.
You can also look at any of the Honda CB350 models because all are comfortable, smooth and spacious bikes which will serve you well.
Another option you can consider is the Hero Xpulse 210. It is going to be the most comfortable machine of all the ones recommended here. Although, do bear in mind it is only sold through the fledgling Hero Premia dealer network. Also, it has some engine reliability issues which have been cropping up of late.



