Alright, thank you for your help. I managed to recover the DB key!

It turns out it was using the same K, but actually -K (so the same R, but with a negative K).

Now, I’ve identified other pairs of signatures (P2PKH addresses) that reuse the same R, but with different private keys, and I don’t know the values of the private keys or K in these cases.
These addresses seem to be related to the ones I just recovered, so likely generated in a similar context.

This time, I suspect that K is large.

So my question is: what would be the best approach to try sweeping these addresses and recovering the private keys?

Thanks in advance for your insights!