macOS Monterey – Custom Notification Sounds for Messages / FaceTime / Mail / Calendar / Reminders?

I want to use my own notification sounds and ringtones for these system apps, just like I do on my iPhone, but none of them will recognize my sound files, and only offer the built-in options (or in the case of Reminders and Calendar, no notification options at all).

I have tried placing my files in these folders:

~/Library/Sounds

~/Library/Audio/Sounds/Alerts

Macintosh HD/Library/Sounds

And while they do show in the system alert list, I cannot use them as notifications or ringtones.

From what I've found by searching around, I'd need to place my sounds in the following folders:

/System/Library/PrivateFrameworks/ToneLibrary.framework/Versions/A/Resources/Ringtones

/System/Library/PrivateFrameworks/ToneLibrary.framework/Versions/A/Resources/AlertTones

But as these are on the System volume, they are read-only directories and nothing can be added or replaced in them, even with SIP disabled.

How can I use my own sounds with macOS, to have them synchronized with my iPhone and iPad?