How to reserve space on a big SDcard for media?

I am going to build yet another OSMC box for use at my summer home. It will be done for an RPi4.
This time I want to use a big SD (128 or 256 GB) in order to have space for media without the hassle of connecting a USB disk as I have done every time until now.

So can someone please tell me how I should go about the install such that OSMC does not grab the full disk space for itself (operating system + kodi)?

If needed I can connect the osmc disk to another Linux box via an USB SDcard reader, but if the osmc image has already expanded to fill the entire SDcard then what?

Note that I don’t want media to reside on the system drive because of backup procedures. The system partition should be as small as possible with some minor space for growth and media should be on a separate partition entirely.

In that case I want to put home on a separate partition because I want to be able to backup by saving the entire system partition without filling it with the media.
This is in response to a reply that vanished while I was writing the reply…
Very strange.
In any case someone asked WHY? and suggested that media should be in $HOME

So install OSMC then shrink the system partition with gparted on your linux box and make a new partition.

You answered my question by editing your OP.