There's already a Baldur's Gate 3 mod that lets you multiclass on Explorer difficulty

When you spin up a new game of Baldur's Gate 3, it immediately asks you to choose between three difficulty options: Explorer, Balanced, and Tactician. The only description for the easiest mode, Explorer, is that it's "a narrative experience placing story before combat." It turns out that choosing Explorer doesn't just make combat in Baldur's Gate 3 easier—it also locks away multiclassing, which is a potentially confusing leveling option for first-time D&D players...
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