What I did in Whonix workstation (No Qubes here since it ran like ass for me)
1. Download SimpleX appimage from website
2. Right click simplex-desktop-x86_64.AppImage and go to properties
3. Click permissions and check program
4. Launch simplex-desktop-x86_64.AppImage
Did you verify the AppImage with PGP? You should always do that. Your way works as well, however, it doesn't have the benefit of automatically updating, and every time you want to upgrade, you have to manually do it and verify with the pgp signature all over again. Your way could be more secure because you use GPG to verify the appimage, but my way is more secure because SimpleX will automatically update itself as soon as an upgrade is released, potentially patching any security holes and bug fixes, before you can do a manual upgrade. Its really a trade off. For the lazy person who doesn't want to pay attention to app updates manually, my way is more secure because it autoupdates. For the ambitious person who want to pay attention to critical updates every time they use the app, your way is more secure.
Question, if you are not running qubes, what are you running Whonix on? I know you can't run it on bare metal, sooooo... Kicksecure? Debian? Windows lol?
Tails was great at the beginning, and got the job done. But then I started to get tired of adjusting all the settings I preferred every time. And then once 24H2 starting peeking its head, I just went Linux almost full time and added Whonix and did some basic hardening. Besides a game here and there, Linux does everything I need! Linux actually does play some of my games pretty well actually. Some, not so much.