BTW, since my projectiles are light I plan to use tandem-warhead navy shells.
Basically imagine a large-bore shell fitted with half shaped charge and a delayed explosive warhead.
This mechanism is very similar how battleship cannons and iron bombs work except they don't need another warhead to do the job for them because the dense iron cap does the job alone.
Shaped charges cost more than a block of iron and of course they take up more place away from the HE charge. They're also more expensive so they're using lower diameter projectiles to save volume which further reduces the explosive power. On the other hand smaller diameter guns can threaten warships and up to its entire range. So you could say death by a thousand cuts.
The low calibers also further discourage Yllendthyr to build large battleships and they only have that as a symbol of prestige.

Does it sound okay by you?
Tandem charges aren't anything really new. They just never required them much. Just like how shaped charges in general sat in the patent library for decades before somebody realized to use them in war.
This is another eccentricity from Yllendthyr which is balanced yet also gives them a certain style.