My advice would be to back up anything you want to keep on whatever media that is handy, reformat the drive(s) APFS and put two, or more, VOLUMES on the drives and forget partitioning. Each of the volumes created on the drive is capable of occupying 100% of the drive's capacity and will expand or contract to hold whatever you put in it as needed. The read/write speeds will be slightly lower but for your intended use it shouldn't make any significant difference. I have two drives configured that way that I use for CCC backups and they are completely reliable and have worked very well.
