Hello Jim!
28 hours from 'Ring to Moscow, in Panda? Very, very ambitious
But my route is like that: in Germany thru Dresden, polish border in Gorlitz, then secondary road thru Bolesławiec to A4 motorway, then to Silesia. In Katowice or Sosnowiec you leave A4 and heading to Warsaw on E75 express road. From Warsaw to Mińsk (Belarus) thru Terespol or Białystok.
In my opinion Poland should take you about 10 hours. You can do a shortcut between Opole and Czestochowa, but on a secondary road, maybe 0,5 hour faster, or from Wrocław to Piotrkow, similar time. And can avoid Warsaw going by TIR transit south of Warsaw, but I doubt it would be faster.
We can pilot you thru Warsaw.
Remember, roads in Poland are much worse than in Germany. I don't know roads in Belarus and Russia, but I think they are no better. It'll be difficult to achieve mean speed faster than 50 mph thru Poland.
I will discuss your route with my friends.
Regards
Sutjeska