yes... anytime...
if you have maxed your farming troops (arch, barb, giants, wb, balloon, gobs) you're good to go...
maxing splash is not required, but it's better because those mortar lv.5 is really good. you might get lv.6 mortars asap you arrive on TH8 because they 1 hit archers.
Don't worry about BK, you can always get him on TH8
no need to upgrade minions either, it's more easier to get DE on TH8



