I want to learn more about adding a signer (signing devices) to Keeper

Please try to use atleast one signer that is a fully verifying signing device. Coldcard and Jade for example. This would ensure that you are able to check the quorum of participating signers, verify address,etc. This is a safety precaution that minimizes threat surfaces to your bitcoin being stolen.

If you plan to give away your bitcoin as inheritance, the Diamond Hands tier of subscription offers you an additional Inheritance Key.