How to mass import personal s/mime certificates

Background: evaluating emclient / migrating from TB with 2 S/Mime affected Mail Accounts. On Install I went with “import from Thunderbird” but while that got the mail account settings, it missed almost all of my personal s/mime certificates.

I have a bundle containing all my current and also no longer valid certificates (that are still required to decrypt old mails). -

Using “Menu, Settings, Signing and Encryption, Certificates and Keys, My certificates/keys, right-click import” to import (500kiB) mycerts_202102.p12 takes about 2 minutes and does not show any new certificates. As I still cannot read older mails (“Encryption has problems” / Local Certificate not found), I guess the import did not work at all.

Is there a way to mass import my personal certificates all at once?

(Once certs are imported I need to assign different certs for encryption/signing to those two mail accounts but that would be another topic)

Edit: env is Win10, might be relevant depending on how certs are handled.