Beginning with macOS 10.15 Catalina, Apple separated the macOS system files from user data. Apple now places the macOS system files onto a read-only APFS volume.
About the read-only system volume in macOS Catalina or later - Apple Support
And beginning with macOS 11.x Big Sur, Apple then made the read-only system volume a signed & sealed volume so that you can be sure nothing is modifying the core system files.
Signed system volume security in iOS, iPadOS, and macOS - Apple Support
What is a signed system volume? - Apple Support
I can appreciate you want to minimize the area for potential issues & vulnerabilities, but macOS is not an OS you can do this with. In fact macOS does not like being modified too far from system defaults (personal experience & also reading these forums). macOS has great built in security especially when the user also practices safe computing habits such as those outlined in this excellent article written by a respected forum contributor (also includes tips to keep the OS running smoothly). While I'm sure you may be already be aware of these safe computing habits, I'm including it here for others who read this thread and want to better understand how to keep their Mac secure & running smoothly.
Effective defenses against malware and other threats - Apple Community
If you want an OS you can customize, then you may want to learn & use Linux or one of the BSDs such as FreeBSD. I was able to customize a Linux installation to only use about 70MB of memory when booted to the Desktop (typically most default Linux distributions' graphical Desktop Environment installs utilize 400MB+ of RAM)....yes it had a very basic graphical interface, but I did lose out on some automated features (not a problem for me). I don't recall how much storage space it took up on the drive, but it wasn't much. Even with Linux, some higher end features may pull in a lot of support libraries and projects. But if you want to customize the OS as you are describing here, then a Linux distribution or one of the BSDs are your best options to do so.
Any unusual customization that macOS used to allow is slowly being removed or broken with each new macOS update & upgrade. macOS is becoming more & more like iOS so expect this trend to continue....even the Mac computer hardware is moving that way too. Some of it is motivated by enhancing security and to make the OS more stable for a better user experience. Part of it is to also keep users from accidentally removing a critical part of the OS. I'm sure other motivations are also involved with some of these changes.
