RAID 1 array with partitions using Disk Utility and Apple Raid Assistant?

I recall seeing some reports on this forum about a year or two ago where people had some issues with APFS on an external RAID which used Hard Drives. I believe they had severe performance issues when using APFS with RAID Hard Drives setup when using Sonoma/Sequoia. I don't know if those issues have been resolved. You may want to perform your own performance testing before committing to either file system.

FYI, I don't recommend partitioning any drive because people almost always realize later on that one or more partitions are too small and to fix the problem usually requires erasing the whole drive & starting over.

The best way would be to use the APFS file system so you could create a second APFS volume which would act a lot like a partition by keeping the data separate & individually mount/unmount each volume independently, but still sharing the same storage pool so that you are not placing any physical limitations on either APFS volume other than the physical drive/RAID size.

Once the RAID 1 is setup with your physical Hard Drives, then you can erase the RAID 1 array with the file system of your choosing, but you need to select the RAID group/array to erase (not sure what it is actually called). It has been a long time since I did anything with RAID on macOS so I forget the specifics.

Also, I hope you also plan to have another external drive/RAID to have a backup of everything stored on your 18TB RAID because RAID is not a backup. RAID is just a way to try & keep that data storage unit running in the event of a drive failure and sometimes to provide a performance boost over a single hard drive. With the huge size of drives today it can take days for to rebuild the RAID array after a drive failure so if another drive fails before the rebuild is complete, then 18TB of data is gone.

It is not easy or cheap when dealing with such large amounts of data.

I prefer hardware RAID, usually RAID-1, -6, or -10, as appropriate. That might include Promise Pegasus and Synology NAS, among other fine options.

If you are using those two non-hardware-RAID disks specifically for Mac backups, I’d run parallel Time Machine backups onto them. Backups alternate targets, with no further housekeeping required.

Given the budget, using NAS and not direct-attached storage is really handy here too, as the backups will run automatically whenever the Mac is within range of the same Wi-Fi network. No cabling required.