This might be a long shot, but if you could capture the Spanish king in battle and ransom him, you should get about 10-15 k in cash. That would help your economy. If you know where he is, and he happens to be in an almost isolated province attack him and the provinces around him with all the troops you can throw at him. He will have nowhere to retreat and (hopefully) you should capture him. Since he is building ships, he should have cash for ransom.

Re: the Russian provinces. I would not have given them up as that way you lose as much income as you save in troop expenses, or even more (e.g. income 200 vs. upkeep 50 or 37). The steppes tend to become loyal to you after a while, so one unit of spearmen/peasants should suffice for garrison duty. Send bishops/spies to increase loyalty. Moreover, the steppes are easy to defend with armies in Khazar, Kiev and Lithuania. And if rebellions occur, crush them and take the ransom for captured rebels.

But of course it is easy to give advice like this, since I'm not playing the game.