Was able to beat on hard level in 5. Mathematically, can't be surpassed.
Turns 3 and 4 were the toughest. Made it to 15 bases on 3 and had to defeat all other nations on 4. 5 was just a mop up of the rest.
OK, now we need a tribute button on the main page under the buffs button. That would effectively collect tribute from all conquered in all colonies without entering the military screen. You should logically be able to do this from a colony that doesn't have a military.