I run Hestia for our development side of things, we dont really run things as the super user/admin user but use regular accounts. While I haven't installed Flarum on Hestia I cannot imagine its all that difficult
create a Hestia user, goto that user account (via control panel) create a database. Ideally you will want to login via ssh at least once as Hestia can be painful with webroot what I wrote was an ssh script that does a ton of lifting but basically for abnormal webroots (i.e the public one for flarum) I would delete public_html (the folder) then
ln -s /home/user/web/domain.com/flarum/public /home/user/web/domain.com/public_html
then chown the lot to the appropriate user
chown -R user:user /home/user/web/domain.com/flarum/
user will be the user you run as, if you insist on doing it as the admin user then if I recall Hestia default admin is admin so .. that, we do it twice so it applies to user and user-group tho if I recall chown user: /path/ works if you want same user/group.
Composer will ALWAYS complain (rightfully so) if you are running it as a super user user/group (chown) fix should at least get your install working but as a super user im not convinced you won't hit problems in the future...
Hestia is a solid panel but try not to put anything under the admin user if you can avoid it.
