All my gpg keys are stored on smart card or on a Yubikey. It is the reason, why I never installed a GPG app on my iPhone, because the Hardware Security Modules were not yet supported. I ordered a Yubikey 5Ci, first mobile applications exist already. Therefore, I would appreciate, if you could may consider to support Yubikey 5Ci on mobile gpg app.
Supporting NFC tokens requires implementing secure messaging for cards. This is on our todo list anyway but has had no priority. I have a couple of Yubikeys but not done any work on NFC.
The new Yubikey 5Ci does NOT work with NFC, this is wrong. This Yubikey is delivered with two connectors: A lightning and an USB-C, see: https://www.mtrix.de/shop/yubikey-5ci/. The key can be connected to a laptop and an iPhone by plug-in. So the new Yubikey 5Ci does not require NFC at all. You refer to the Yubikey 5 NFC. This technology is not supported by developers because they do not have experiences there. With the plug and play functionality of a lightning connector it is easier and few application already exist (e.g. Yubico authenticator and several password manager in the professional edition). Hope this information will be useful for you.