Not present doesn’t necessarily indicate a Whonix or arm64 port specific issue. Unless available in current Whonix releases.
Onionshare is written in python. Hence, most likely architecture dependent. Please check if these applications work on Debian (buster) arm64 first. See also beginning from here: OnionShare Whonix integration development discussion - #27 by Patrick
Debian -- Details of package thunderbird in buster shows arm64
. I would expect Thunderbird to be installable as per usual in Debian.
Also FYI: