I apparently made the “mistake” of changing my SSH key password using Seahorse, which stopped it from automatically unlocking (my SSH connections always got stuck at debug3: sign_and_send_pubkey: signing using rsa-sha2-256 SHA256:). I tried everything else without any success in getting the password to be remembered, but this worked. Thank you so much.
For id_rsa the command was:
secret-tool store --label="PW id_rsa" unique ssh-store:$HOME/.ssh/id_rsa
And then I confirmed the password is remembered by running this command, which I had been using to check whether the various fixes actually remembered the password:
systemctl --user restart gcr-ssh-agent.socket gcr-ssh-agent.service