I’m sure there’s probably some sort of mdadm invocation to shrink an existing RAID1 mirror pair, but in the absence of that, one option is to create the mirror pair you want on your 2.5" SSDs, and then shrink-and-clone the filesystem from the NVMe mirror to the 2.5" SSDs, using something like Gparted.

In that way, you end up with the mdadm pair that you wanted, and the filesystem is correctly sized.