As I frequently tell people, there are no unicorns. A bigger bike will make long road stretches easy and carry luggage effortlessly. But it will also be harder on chains, tyres and fuel, be a handful off road and be problematic to pick up when it falls over. The converse is true for a small bike.

To me, 700-900cc is the sweet spot for a mainly road based adventure tour. Maybe down to 300 or 500 if you're anticipating a lot of offroad action. No traveller ever said "I wish I'd had more expense/weight/power on that tour."

The T7 is a great bike (I used to have one) but I sold it because I wasn't happy about the top heaviness. I ride a 790 KTM now and love its performance and low centre of gravity, but not everybody likes KTMs. Consider also the Honda Transalp, it feels lower and more rider friendly than the T7 but you would have to spend a bit on accessories to "adventurise" it.

Re your journey. Mongolia is certainly achievable but there are parts of the route to India that could be problematic. I'll let others speak about those as I'm not an expert there. If you're after a 6 months ride then Mongolia would fit your bill. You could ride there directly through Russia then on the way back drop down to Kazakhstan, Kyrgyzstan and Tajikistan which offer incredible landscapes and history. Thence to the north of the Caspian Sea to Georgia and Turkey back to Europe. Enter Russia via Latvia (not Belarus and obviously not Ukraine). Or go via Georgia both ways to minimise time in Russia.

Russian visas are available and tourists are welcomed as guests. Obviously be sensitive to the situation but the ordinary people aren't the government and are very hospitable. Take enough cash to cover your stay, western cards don't work there, and be prepared to buy medical insurance in the first city you come to, for the same reasons.

Note some of the high passes in KG and TJ may be snowbound up to June.