Can't boot a 3 x 4TB disk RAID 5 on a UEFI system?

[C5ace]

I sold my 12 year old PC booting of a 4 x 1TB HDDs RAID 5 software raid (mdadm) using grub-static. The /boot ext4 array is RAID 1. The / and /home are ext4 RAID 5 arrays. Swap is also a RAID 5 array. DOS partition table. This has worked for reliably for more then 10 years, including replacing 1 failed HDD and adding one from 3 to 4 HDD's

My new PC with UEFI has 3 HDD's of 4TB each.
I tried Grub2 and GTP partition table. Could not get it to work.

Question:
How can I set up the new system like above so it boot if any of the 3 disks fails, without adding additional points of failures and complications like LVM, BTRF, USB boot sticks, etc.
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Observation after 30 years working with computers:
All software has known and unknown bugs and vulnerabilities. Especially software written in complex, unstable and object oriented languages such as perl, python, C++, C#, Rust and the likes.