Is there a way to adjust the speed of the Photos screensaver (slideshow) in macOS Big Sur and later?

I find the slideshow speed too fast and would prefer a 15-30 second duration. I know the relevant plist is at /System/Library/PrivateFrameworks/Slideshows.framework/Versions/A/Resources/Content/EffectDescriptions.plist and I believe the entry that needs changing is mainDuration in the JustASlide key. I tried recursively copying /System/Library/PrivateFrameworks/Slideshows.framework/ to /Library/PrivateFrameworks/Slideshows.framework/ and then changing the destination plist, on the hunch that user library settings would override System library settings. This had no noticeable effect. I've found the below Q&As helpful in researching the issue, but I do not want to disable SIP and authenticated root and therefore be unable to re-enable FileVault. https://apple.stackexchange.com/questions/135766/os-x-mavericks-screensaver-changes-pictures-too-quickly-how-do-i-slow-it-down https://apple.stackexchange.com/questions/378905/how-do-i-change-the-image-display-duration-in-the-catalina-screensavers https://apple.stackexchange.com/questions/395508/can-i-mount-the-root-system-filesystem-as-writable-in-big-sur