A one-day specialized course on lithium batteries would be sufficient.
However, it is advisable that you should have at least one of your staff take a full 4-day Category 6 course. This person will be able to function as a mentor in your company who can give guidance to any DG matters. As regards to who should sign the Dangerous Goods Declaration, this is an internal administrative matter.
It does not mean that the person who succeeded in the DG training is authorized to sign the Declaration. The Declaration is an outgoing official document, and the person who places the signature need to be an official representative of the company. It could be the president, vice president, section director, section manager, section supervisor as determined in the administrative procedures of the company.
Notwithstanding whose signature appears on the DGD, the Declaration must be prepared by the person who have been qualified through training.