How can I remember the SSH key password on a remote system that SSH's into a second remote system?
Solution 1:
This answer provided me with a solution.
By using ssh -A user@new-mbp and then ssh user@raspi I re-use the ssh-agent of the New MBP (if I understand it correctly) on the Old MBP and can login on the Raspberry Pi using an SSH key without being prompted for a password.