this is still wrong or at least bad practise. your webserver should not run your page and scripts as www-data in the first place but instead properly switch to the user you created in hestiacp and where the files are under. the owner and group of all files should belong to this user by default and therefore it is important that you run the installation of magento from that user to begin with.
changing the group to www-data and allowing write permissions to that group is merely a workaround but really not a good one. think about environments where you would have different customers share the server. if everyone runs as www-data you can actually cross connect/access foreign data and that’s a big security risk. just saying…
congrats on getting it up and running though! and thanks for documenting as well, I think there will be more people looking for that info 