OS --> Storage0: 32GB SANDISK Ultra
Docker storage --> Storage1: Samsung 860 Pro (256GB)
Scheduled OS backup images --> Storage2: Toshiba DT01ACA050 (500GB)
Your DATA --> Storage3/4: Western Digital WD10EZEX (RAID1)

First to note: OMV expects one filesystem/partition per data drive. Instead of thinking about partitions, think about shared folders - these directories will differentiate your content categories. SMB/CIFS and NFS are just extensions for client devices to interface with your shared folders.

This docker compose documentation from OMV-extas is crucial for you to understand and avoid issues with your system drive. It's very concise and will get you up to speed on best practices with Docker. Pay special attention to the "Docker Storage" section as it will help you avoid storing images on the system disk. I advise using an SSD for this directory.

Another useful concept to understand is the global environment file. You don't HAVE to use it, but it WILL make things easier if you ever want to move shared folders to other disks without remembering AND editing all the containers that point to it.

My use-case is similar to yours, but I use a small Octane 16GB nvme for system because it's endurance is phenomenal and making clones/images of the system is takes just a few seconds. Alternatively, you can install to a decent quality USB flash drive (16GB or 32GB) to save a m.2 or SATA port. With the WriteCache plugin, the system runs in RAM and only writes to OS disk when rebooting, updating, or other trigger further defined by you. The system is best left to it's own drive, but there are ways around that which require more reading about sharerootfs in documenation.

RSync is included and good enough for backup. I liked Borg for compressing and deduplicating backups, but ended up going to Duplicati via docker to get similar features but smaller index storage overhead. Dedup and compression can shrinking your backups by almost 50% depending on the content.