gnupg2.2.4 in gpgsm.texi has

@item --export-secret-key-p12 @var{key-id}
@opindex export-secret-key-p12
Export the private key and the certificate identified by @var{key-id} in
a PKCS#12 format. When used with the @code{--armor} option a few
informational lines are prepended to the output.  Note, that the PKCS#12
format is not very secure and this command is only provided if there is
no other way to exchange the private key. (@xref{option --p12-charset}.)

for readers it would be good to point out how the mentioned exchange of the private key should work instead
and in a more secure way. I also haven't found a description for this in gpg.texi.

Use-cases are:

• Want to move the pub- and privkeys to a second machine

I guess that for this use case, copying the relevant file from ~/.gnupg/private-keys-v1.d/ ( $GNUPGHOME/private-keys-v1.d/) from one machine to the other shall work. Is this correct?
(Readers could profit from knowing how to identify the privkey file in that directory).