This is not an option for me. (using 4.2.4 installer)
I did this in the past here: Unable to get fully functional system when installing Qubes 4.1 on a RAID1
But I found that ext4 in a RAID1 seemed to prevent me from doing backups in Qubes. I’m assuming you are not having this problem in 4.2? The only filesystem that was useable for my system was XFS at the time. BTRFS was too slow.
Following my instructions at the link above seems to only work with MBR partitioning, not GPT. And GPT is recommended for RAID and UEFI from what I’ve read. And GPT is what requires creating the separate partition for /boot/efi - yet as I said above, I don’t have that option in my installer.
Any advice?