One should take as much time as possible for such a journey. The two months that were available to me are actually too short. That makes such a journey very exhausting.
Everywhere the people I have met were very friendly, even though the language barrier is big, when you do not speak Russian (like me). So much the better if you can speak some Russian!
For communication in Russian, I often used Google Translate. For that reason alone, you should get a local SIM card (see next bullet point). With the language function you let Google Translate speak for you or understand what people are talking to you, and in the supermarket you can sometimes try the function for image translation of Google Translate to find out what is actually in a pack. While this is still far away from the "Babel fish" that Douglas Adams invented in "The Hitchhiker's Guide to the Galaxy," it's always helpful.
I bought a SIM card with Internet in most countries. This is usually much cheaper than in Germany/Europe. Important: If you want to use your mobile phone as an WIFI access point for your computer, then you should always ask if the Internet of the SIM card also works as "hotspot". Sometimes you have to pay a small surcharge for this feature. Always insert the SIM card in the shop (or better let the employees do it for you). Don't forget the tool for opening the SIM card slot, if needed for your phone. After replacing the SIM card, try it out right away. For To increasing the amount of your credit on the SIM, you may go into some mobile phone shop. I only had to top up my credit in Russia, where you easily can do it on many ATMs with your Visa card.
For such a trip you should be prepared quite well. You do not necessarily have to be physically fit, but to ride such a long distance in such a short time is very exhausting! You should be aware of that!
You should prepare your vehicle very well: For this reason you should know the usual problems the vehicle might have and then either have them previously revised in a workshop or take appropriate spare parts and tools and also know how to use them. On the way you will always find help, but for many spare parts that does not apply.
Anyone who wants to ride this route by motorbike should have some offroad experience before. This is not necessarily a must, especially if you are riding as a group, but it helps a lot.
Luggage must be stored very well in the panniers, the vibrations on the ride are incredible and disassemble anything that is not nailed down.
I would always try to get visas before the trip starts. For Russia, I applied for a business visa, which is quite expensive (200 euros), but I would have been too annoyed to try to apply for another tourist visa for Russia in Dushanbe or elsewhere. For Uzbekistan, I applied for a 30-day visa, even if it was 20 euros more, although a 14-day visa would have been sufficient. But if you have a breakdown somewhere or you are stopped by other circumstances, then it is nice, if you do not immediately run into problems with the validity period of visas for the countries, you still want to travel to. This safety is worth the 20 euros more in any case.
I did not pay anything for bribing or penalty. Maybe I was just lucky, but you know that the police in Kyrgyzstan and Tajikistan is trying to get cash (US$) from you and you easily find out that they are checking speed in places where there are signs with speed limits of which not so many are existing in those countries. So just take care in these places!
And if you get caught, don't be so easily intimidated. In Tajikistan and Kyrgyzstan and actually in the other countries, I would never give the originals of the driver's license or vehicle registration certificate to a police officer. For this reason I had (laminated) copies with me. I got rid of one. When a policeman asked for the original, I answered him in German that he will not get the original. He kept the copy of the license and did send me away. So both sides could save their face ;-)
There are many permanent control stations in some of the countries. Most officials only want to see the travel documents. For all other controls, I was always careful, first slowly took off gloves, slowly dropped down the helmet, slowly removed the earplugs. Then you can already assess the situation much better and sometimes the officials are already in the middle of talking about the motorcycle. Always stay friendly and laugh/smile. Do shake hands with all officials nearby. Do not understand too much Russian and do some jokes. That always worked for me.
I have had good experiences with the 1plus Visa card from Santander. You really do not need to bring any cash except for Uzbekistan. Having a few US dollars is helpful though. For Uzbekistan you need US dollars (cash).
However, especially for Mongolia and the Stans: timely get cash from ATMs. ATMs are not everywhere and not everyoen will work with your credit card, even there's a sign for it at the ATM. Pay attention to holidays! Sometimes ATMs are in the bank and therefore can be used during bank working hours. If no ATM works, then there is always someone, who at least will exchange US dollars (perhaps Euro too, but US$ are better), e.g. the hotels.
And at least: Riding to the East you should be aware, that you will loose time every day, see also here.