Thanks, that clears up the part I was missing. Using your numbers, the jump from a knight’s 18 upkeep to a crusader’s 54 is another 36 each round.

For anyone trying the seed, that’s a useful distinction: having enough saved to buy a stronger unit doesn’t mean that connected territory can keep paying for it. Checking that territory’s income and upkeep before committing seems like a good way to understand the starvation problem you described.