There's no perfect solution here. If admins can't edit other admins, who would create or remove additional admins ?
Flarum has no concept of super-admin or root admin. All admins are equals. I believe there's only logic to prevent deleting our own user, as to prevent deleting the last only admin.
The next question is do you really need multiple admins ? The admin role is extremely powerful in Flarum, even more so if you have the package manager installed, as they will be able to effectively run arbitrary code on the server. Maybe your use case would be best served with a single admin, and then use moderator roles for all other admins. You can rename those moderator groups to be identical to the admin badge visually if you need.
If you really need to give multiple users access to the admin panel and need to prevent abuse, you will need some kind of lock-down extension anyway, that hooks into the various admin features and disables the most dangerous ones. If you only want to prevent editing other admins users, you might be able to achieve that with a local extender.
Personally, I have designed all my extensions so that moderation features that you might want to delegate are not placed in the admin panel, but instead in the main UI and controlled by a permission that can be given to any group. Most extensions do this, and it shouldn't be too difficult to modify an extension if you need to separate and permission-restrict a feature.
If an admin account is compromised, you will probably have to directly access the database anyway. You can easily remove or re-add the admin role to any account through there.
