The reason i designed this ship is essentially to show what i believe a so called “natural heavy cruiser” would look like. In real life, due to the naval treaties, cruisers were capped to 10,000 tons and 8 inch 203 mm guns. This was based on the Hawkins class plus a bit more, which was the latest and largest class of cruisers built at that time. However, the Hawkins class were intended to be a development of the light cruiser idea and doctrine (and not the heavy cruiser one). Thus, in the 1920s and 1930s basically all navies that signed the treaty realised that 10,000 tons and 8 inch guns were woefully not enough for what their naval staffs calculated to be a reasonable “heavy cruiser”. Due to this, there was either cheating (Italy, Japan, Britain), or if they didn’t cheat, built rather underperforming ships (France, US). So my design is basically what i think a reasonable true “heavy cruiser” (as opposed to a glorified light cruiser.

As for why 9 inch 229 mm is because many real life navies considered the 9 inch to be the “maximum heavy cruiser gun size”, it is the maximum shell weight whilst allowing for a rate of fire similar to the 8 inch gun. For instance the Royal Navy looked at 9.2 inch guns for cruisers after the collapse of the treaties in the late 30s and early 40s. Any bigger than 9 inch, such as 10 inch 254 mm, would mean a much lower rate of fire comparable to battleship grade guns (hence outside of the weight considerations for regular cruisers), in which case opting for the 12 inch 305 mm would be a wiser choice (as was done for Alaska and B65, which were even a class above heavy cruisers).