How did you manage? Didn't you get spammed every damn day with massive revolts in random provinces?
I turned off alerts about rebels and battles, and automated my armies for rebel hunting. I also had a gigantic army at full maintenance fighting them - IIRC it was between 500,000 and 600,000. In sufficiently large stacks, they almost never were vanquished, and it was late enough in the game that supply limits were high.
However, it was rather boring. My stability was at -3 all the time. At first that was because of very frequent stability hits due to uber-infamy, but it stayed there because it cost about a gazillion ducats (actually more like 78,000?) to gain a stability point, because of a gigantic increase to costs because of infamy. So, obviously, I didn't try to invest in stability. When there were wars, I'd just take a hundred thousand troops or so for foreign wars, and leave the rest bashing rebels. But a lot of the time there weren't wars, as after being beaten thoroughly a few times, my neighbors stopped declaring war.
My tech level did fall from top-notch to middling-among-Latins because of this, though. Not so much because of inflation at 20%, but because my economy tanked without trade income and with considerably lower tax revenue.
Well, I have National Bank and Mint Master (reduces 0.12), in total would be 0.22, that's quite a lot, but, it doesn't serve me. I am in the year 1423, so I must start a new game. Too bad.
Well, not necessarily. The
trajectory is quite bad - you are on pace for 176% inflation by the end of the game, which would be quite impressive. However, you aren't in dangerous territory yet - you just can't keep having it rise at that rate for too many more decades. 10% wouldn't concern me, and after all, you've probably gained some benefits from that. I've played with 20% for an extended time before, and it isn't that noticeable (harder to lead in tech, but you won't be left in the dust). The issue is the rate isn't sustainable - you'd have almost 42% in 1500, which would be having a major economic/technological impact (and at that point, also make it considerably harder to keep it in check).
Also, although I haven't played as Portugal or Castille, I'm pretty sure Castille starts off in a much stronger position. Your economic stewardship may be better, too, but it'll probably look quite a bit better regardless as Castille.