Don’t get me wrong, but that is stinky peace of crap, not the single file standalone executable. I use AppImages all the time and they are 100% standalone single file executables. I can have a subsystem to make my life easier, but I don’t have to - they still work out of the box just downloaded.
Did I miss something about snaps? Is there any other way to have todoist if snap thing is overcomplicated?
You are running kde, if I remember correctly kde has the snap plugin already installed for pamac (add and remove software) so all you need to do is enable snap in the pamac gui settings to install snaps. But as @pablomhr said you can install it from the AUR. All you need to do is enable aur in pamac settings
Snap and flatpak are sort of “sandboxed” apps. They work the same as official apps but contain all the necessary depencies so tend to be larger files. Its always best to use official repo apps first but I run some Snap packages as well