I read through the documentation when I was labbing proxmox, and it specifically calls out that if you need replication the only way to do it is to not use a raid controller (present the drive's directly to the OS).
More specifically, they intentionally restrict it to ZFS. Which really, really sucks. ZFS is a waste of overcomplication.
Unless there has been a lot of turnover in Proxmox management, this is still the same company that insisted software RAID was unusable till they decided to adobt ZFS. Doing things weirdly and/or more complicated than it needs to be is just what they do.