Well there are two ways for the axis to win; either take two capitals or get an economic victory. Yes, my plan would be to go after Russia. I think 21 ipc might be a bit low to do this (I might give it a try for fun though).
Taking the US or UK isn’t realistically going to happen without acheiving what would be an economic victory anyway; so what is the best way for the Axis to do this - Russia or Africa?
In my opinion, Africa is not rich enough to justify a massive Axis attempt to hold in the early game - especially as the US/UK are always going to be able to send more resources there than the Axis. They generally take Africa back before Japan gains enough in Asia for an economic victory. Additionally, if Germany sends too much resources there, the Russians become problematic.
If Russia falls however, then the Axis will soon have all Asia and for an economic victory then the Axis can take a look at Africa again.