The installer (subiquity) supports the "autoinstall" config files embedded in the install USB/ISO if you want. You can also just point the USB/ISO's autoinstall

The installer (subiquity) supports the "autoinstall" config files embedded in the install USB/ISO if you want. You can also just point the USB/ISO's autoinstall to an HTTPS address so an update doesn't require spinning new install media (does require the install start with some form of network access then). If you have wildly technical users only then they can even just have them type the autoinstall web address in the grub entry before the standard install media autoboots the entry (this falls apart the second someone isn't ultra techncial). In any of these cases the autoinstall file can still handle both the installer as well as post install custom scripts to do whatever you need but then it's done. For long term management/enrollment you'd need to look at something beyond just putting stuff on the installer USB/ISO. Ubuntu has Landscape for this but there are some alternatives. Just depends on what you need to manage long term and what you can realistically do (e.g. you note you aren't in a spot to manage something like Puppet).