Again, you don’t need stirrups; Konniks, even without stirrups, still beat knights on a 1v1 basis, even without getting unhorsed. Per cost, they’re even better.
Again, I think your problem is waiting too long to go into them and spending too much on improving them, rather than getting them out right away. You don’t need full infantry upgrades for them to be worth using; they’re already extremely powerful even with just the cavalry upgrades.
Again, the Krepost can be considered to have no effective extra cost, because you’re basically just paying for 4 houses and 1.5 stables, not to mention that free 100 stone you start Castle Age with.
What it sounds like to me is, you’re not going for Kreposts because you want Stirrups, which means you end up spending something on the order of 1550 extra resources before you even get to production, including the infantry upgrades.
You then enter the fight with a ~13 unit disadvantage, and naturally, you lose.