Use what is more comfortable for you. I have done it the classical way on 25 computer in one room. But not Manjaro 
In general I would recommend to setup a mirror server first and use that one for installation and updating. It will speed it up a lot. @moson Has a nice docker here: https://github.com/moson-mo/manjaro-mirror
- Create a bash script which will do the unattended install. It is basically the same like here, but it runs automatically in a script: [root tip] [How To] Do a manual Manjaro installation and maybe you could get some inspiration from here: GitHub - picodotdev/alis: Arch Linux Install Script (or alis, also known as the Arch Linux executable installation guide and wiki) installs an unattended, automated and customized Arch Linux system.
- Put the script on a local web server.
- Modify profile before building the ISO, set it to autologin and let it download + run the script after login: Build Manjaro ISOs with buildiso - Manjaro
- In general you should create a basic iso with just basic stuff on it for booting. (like the architect version)
Maybe there is a tutorial for this, but I have not found any…