Ideas and suggestions.

Building just one fire-resistant bird in the Fortress for 15,000 is not profitable at all.

Well, then don't build it. Birds in the Fortress are not positioned as a main attack force, but as a tactical weapon – speed, immunity after upgrade. Accordingly, they are supposed to be built in the 2nd week, and in the first week, you should max out low-level melee units. Dragons have a similar role.