You don’t need to manually create subvolumes. The installer will automatically create them using the subvolumes and mount points defined in /etc/calamares/modules/mount.conf.
If you want to change the default subvolume setup, for example to rename the subvolumes or add additional subvolumes, just edit the file before you run the installer.
The installer needs to set up new subvolumes. If you want to reuse an old subvolume, do it after the installation is complete (for example, remove the @home subvolume the installer sets up, then rename your old home subvolume @home).
Those are two very easy ways. Probably there are other ways, but those two methods are simple, easy to understand, and fast.
I don’t think it is the majority of users, I think it is a relatively uncommon installation technique. More typically people will make a backup of important data from their old installation, completely wipe the disk when they run the installer, and restore whatever files they need from the backup afterward.
Go ahead and write one. If it’s good, maybe it can become a Discovery article. https://discovery.endeavouros.com/ Be sure to take notes as you go. 
Probably most Linux installers try their hardest to not be like Anaconda! 
I think it has gotten better over the years, but I have always found Anaconda a bit clunky and unintuitive.
Adding a feature like that is a bit outside my wheelhouse, but if you’d like to take a crack at it you can learn about contributing to the EOS Calamares implementation here: https://github.com/endeavouros-team/calamares/blob/calamares/CONTRIBUTING.md