Looks like that NAS was originally sold with up to 40tb capacity so it shouldn’t have any issues with larger drives. Seems like the “my cloud os” is based on Linux so unless WD built in some weird limit, it should work with 20tb drives.

I don’t have an answer here, never had to rebuild an array. You might be able to use clonezilla which can do a block by block copy of disks and then expand the volume in the OS if it supports it. This is just conjecture, I’ve never done it with a raid array.