Add nice setting or command line option

Is your feature request related to a problem? Please describe.
Creating a timeshift snapshot can at times use up a significant amount of resources. Since the process is run as root with a nice value of 0, it takes priority over all other processes running under the user which includes almost all parts of the UI that the user directly interacts with.
Setting the parent process's nice value does not appear to do anything, nor does launching timeshift with nice. Opening plasma-systemmonitor to check the process's nice value reveals that it's automatically resetting it's value back to 0 for some reason.

Describe the solution you'd like
Either an option to specify a custom nice value or a way to ensure timeshift keeps it's initial nice value would be greatly appreciated.

Describe alternatives you've considered
I could try to modify my personal backup script to launch timeshift in the background, wait a couple seconds and then modify the nice value of all rsync processes.
This isn't ideal as it wouldn't be guaranteed to always work and it may also change the nice value of other unrelated rsync processes that are running in that moment.
This solution also wouldn't work for anyone who intends to backup from the GUI nor would it work for anyone who intends to use the built-in automatic backup system.

Additional context
I haven't tested if ionice faces a similar issue or not, as I backup to an external disk.

Start by tracing how Timeshift launches rsync for GUI and automatic snapshots, then inspect where process priority is set or reset. Done means the requested nice-value behavior is supported consistently for snapshot runs without affecting unrelated rsync processes.