Proxmox is not supported for Software (FAKE) RAID.
The only other solution is to install Debian, and then install Proxmox on top of that, but this is NOT recommended.
I am in a similar situation with 4 - Blade servers that cannot hold hardware RAID cards. I am looking into a possible solution of copying the disc with clonezilla/cron at intervals, and using shared storage for VM/CT
I do not recommend the software RAID solution, as I have seen several forum post that show irrepairable breaks after updates, and they will not be bugs, because software RAID is not supported
