How to install a second startup system without copying the entire disk (especially the applications ?)

There are basically two methods

On way is On the existing Internal Drive for more realistic Performance testing

The second way is on a External SSD Drive at lower performance testing

#1 Use more than one version of macOS on a Mac - Apple Support

# 2 Use an external SSD as your startup disk with your iMac or Mac mini

When a Long Time Contributor like @BDAqua joints this question of yours.

I usually sit up and pay close attention to the advise .

I too would probable use the external SSD Drive is Testing Sonoma.

Just so you understand where I am coming from.

Have upgraded 2 M1 Computers from macOS 11 Big Sur to macOS 12 Monterey to macOS 13 Ventura and now on macOS 14 Sonoma 14.0

I keep my machines ultra clean of Third Party Software and all upGrades have gone without issue apart from learning the Change from System Preferences to System Setting in macOS 13 Ventura and above

Some Users have had success Cloning to a External Drive and then upGrading the External to Sonoma

Other have not

Personal opinion not solicited - Install the Sonoma Directly to the External Drive to avoid any disappoints or added complications

Worst Case if it fails - Erase the External Drive and use the earlier suggestion above

The Apple Apps aren't likely to work in more than one version, I'd go with installing on an external drive. preferably an SSD.

I did exactly that on a clone of my Venture system to an external SSD. I used SuperDuper! to make the bootable Ventura clone, booted from that clone, then upgraded it to Sonoma.

Keep us in the plural updated on your progress