Looks great!
I wonder if there is a way to convert the nsec to a Bitcoin seed and back?
Discussion
You cannot convert it into a Bitcoin *seed*. You can, however, simply convert the npub into a p2tr address and vice versa, since they are both created the very same way.
You can derive bip32 seed words and use these to recover, but I think it would be more fun to use hierarchical deterministic derivations instead of just a single nsec