Difficulty is mainly judged by three factors: What the best similar example of a technology you have built is, how long ago you built it, and what progress the outside world has made in that area
The Cannalan's had decades of experience using multiple ship designs. We have a year using two.
To suggest both our experiences are now similar is silly.
The Cannalan's could not manage what you're planning, and the cost of their design is significantly bigger than ours. It rolled a 1 yes, but a roll of 1 is not a complete failure in this game. It just results in a fatal flaw, which they fixed with a revision.
If we burn a design on a wastefully simple upgrade because we're acting too scared to do what needs to be done, we lose just as surely as if we were to ditch our focus on aircraft and design an all-gun navy. We CAN'T beat them if we don't make advances, and we can't beat them if we try to match them in WHAT THEY DO BEST.
Nobody is suggesting an all-gun navy.
We're talking about fixing the gaping hole in our fleet composition...
What hole does a 300mm fill that our aircraft don't, and better? There's no need for bigger artillery, our planes are good at sinking their ships and striking targets from over the horizon, the problem is the enemy having parity because they can launch more planes than us. To me that doesn't say "new cruiser", which doesn't address the enemy's air advantage, it tries to fight their already well-established artillery advantage. They can easily negate a cruiser design by improving their torpedo bombers and new ships do not stop bombers, Combat Air Patrols do.
Our planes stop their ships dead as long as they're not outnumbered.
If they can't harass a Khorne by setting its crew on fire with incendiary bombs and destroying its turrets, they can't stop a Khorne from destroying two Archer destroyers in a single volley.
"During WWII air raids accounted for the loss of warships and merchant vessels of all types, including the battleships Conte di Cavour, Arizona, Utah, Oklahoma, Prince of Wales, Roma, Musashi, Tirpitz, Yamato, Schleswig-Holstein, Impero, Lemnos, Kilkis, Ise and Hyūga."
In the era of air superiority battleships don't sink battleships, planes do.