There's no point having cities with all the improvements destroyed.
I suggest switching to Slavery
& Nationhood (Drafting). It'll take three turns of anarchy but probably worth it. Either one of them will take two turns anyway. If you only want to do one then I suggest taking Nationhood over Slavery as that only give unhappiness.
Move the grens, cannons, pikes and spears from Birka towards Bagacum. One turns takes them to a hill. Move the cannons ,spear and knight from Camulodunum towards Bagacum. One turn also conveniently takes them to a hill.
I think you should probably have started doing some of this reinforcement earlier but I understand the reluctance to attack Izzy's stack.
As far as Ghandhi goes it may be worth evacuating New Karakorum of the grens and then attacking with the chariot so that the cavalry take it instead of pillaging. Just pray he doesn't raze it. Alternatively reinforce from New Sarai & Napa Valley. We just don't want Ghandhi razing all those lovely towns.
EDIT: Another possibility would be to upload your latest save to this thread so that we can have a look at it to see exactly where everything is.