New item: potion of boost knowledge
A new item that would trigger skill level increases.
I would suggest a new, rather rare, potion to the game, called for example Potion of boost knowledge. Its effect would be allowing a skill level increase event similar to when leveling up. At some point skill leveling becomes scarce, making for example bridge building and late-acquired swimming hard to train, and this would allow some tactical choices to be made. The effect could be for example:
blessed: as many skill increases as Le score allows
uncursed: half (round up) skill increases based on Le
cursed: one skill increase
The potion could be rather rare, maybe similar to potion of gain attributes. If further nerfs are needed, a wish could yield only one of these and only one could be dipped at a time. But it could be involved in possible Alchemy recipes.