saturneric
Signatures - signing subkey selection
You might want to consider that keys can have several subkeys eligible for signing so that a key selection could show subkeys, see image of how WinPT for GPG legacy handles this.
After some research on my part, the programming interface provided by GnuPG (GPGME) does not currently support specifying a specific subkey in an operation. However, GnuPG itself supports it with its command options. Things get a little more complicated, and I don't really want to go directly to GPG and operate through the interface they provide.
So, this requirement may be placed for a while longer. I may find time to provide them with feedback and expect them to provide such support.
Now with v2.1.6, you can see the best practice of subkeys: export it, and use it separately without the primary key. And you will realize that the function of selecting a subkey for a particular operation is not a must, at least for now.
Refer: https://gpgfrontend.bktus.com/guides/generate-use-subkey
With recent updates, GPGME has added the ability to specify subkeys for signing operations. As a result, GpgFrontend now supports selecting a specific signing subkey when multiple keys are selected and some of them contain additional signing-capable subkeys. When this situation occurs, you can now choose exactly which subkey(s) to use for signing, making the workflow more flexible and in line with best practices. You can try this feature in the latest Nightly Build.
Feedback and further suggestions are always welcome!