I'd definitely say two things are needed in regards to your economy

The first: I would secure a vassal/alliance by any means necessary (other than hostage). Once you hit realm divide, Japan will hear the call of the shogun. Any trade relations you have going will go right down the drain. If you have a vassal or an ally with some good standing, you can keep trading for a while.

Secondly: You need to be self-reliant. That means farm upgrades. lots of farm upgrades. When i neared RD, i started teching up. I had a level 3 farm in every province, two trade nodes and two vassals, and barely managed to break even (with a two full modernized stacks). As i secured more territory and expanded my trade dominance, this margin quickly increased. But those first years? Don't be surprised if you make -2k a turn. You might find yourself disbanding a lot just to keep yourself afloat.