For email that is GPG signed or GPG encrypted I store my private key on the smart card with my Yubikey. I have it properly configured and integrated with the host gnupg packages.
I found with the thunderbird snap however, even when I follow the steps to allow external authentication it doesn’t work.
Performing the exact same steps on thunderbird package from Ubuntu 20.04 repositories I have no problems. So I’m suspicious that this is related to confinement rules for the Thunderbird snap and will need a way to break out to use the host gnupg.