You don’t need a RAID controller, I have dual NVME set up with RAID1 and boot off the RAID one partition, the only partition I can’t raid is the EFI partition because BIOS doesn’t know about it, but that I simply duplicate by hand on both drives using dd, since it only gets updated at kernel updates, it just adds a dd to the kernel upgrade process.
Can you do it? Yes. Will the performance suck outright? Yes. Nextcloud is a pig. I run it on an 18-core i9-10980xe server clocked at 4.5 Ghz with 256GB of RAM, with RAIDED nvme disk, and I don’t find the performance adequate on this platform.