So this is what I get when I load OS on the SSD. I did not mount our do anything to the sata drives. It looks like or is it logical to raid them first or better off using them separated and mount 1 drive?
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 3.7T 0 disk
sdb 8:16 0 3.7T 0 disk
nvme0n1 259:0 0 477G 0 disk
├─nvme0n1p1 259:1 0 511M 0 part /boot/efi
├─nvme0n1p2 259:2 0 511M 0 part
│ └─md2 9:2 0 511M 0 raid1 /boot
├─nvme0n1p3 259:3 0 475.4G 0 part
│ └─md3 9:3 0 475.4G 0 raid1 /
└─nvme0n1p4 259:4 0 511M 0 part [SWAP]
nvme1n1 259:5 0 477G 0 disk
├─nvme1n1p1 259:6 0 511M 0 part
├─nvme1n1p2 259:7 0 511M 0 part
│ └─md2 9:2 0 511M 0 raid1 /boot
├─nvme1n1p3 259:8 0 475.4G 0 part
│ └─md3 9:3 0 475.4G 0 raid1 /
└─nvme1n1p4 259:9 0 511M 0 part [SWAP]
Filesystem Size Used Avail Use% Mounted on
udev 63G 0 63G 0% /dev
tmpfs 13G 884K 13G 1% /run
/dev/md3 468G 1.9G 443G 1% /
tmpfs 63G 0 63G 0% /dev/shm
tmpfs 5.0M 0 5.0M 0% /run/lock
tmpfs 63G 0 63G 0% /sys/fs/cgroup
/dev/md2 487M 80M 378M 18% /boot
/dev/nvme0n1p1 510M 6.1M 504M 2% /boot/efi
tmpfs 13G 0 13G 0% /run/user/0
OK so it looks like you actually have 4 drives total. 2 platters and 2 nvme's. the raid 1 is fine with the nvme's. now you need to make another softraid for sda and sdb. put that in raid 0. after you do that, format it to something like ext4. and mount it. once you're sure everything is good, add it to fstab. and give it a reboot to make sure it auto mounts.
Bonus material:
Once you have everything all working and auto mounting, you can move /home to /home_old, unmount that platter raid and mount it back as /home. copy the contents of /home_old to /home and edit your fstab to persist the new mountpoint. reboot again and boom, now you've got your OS on a raid 1 nvme array, and you have your /home on your 8 Tb raid 0 array. keep in mind that IF one of the platter drives fails, you will loose everything. 8 Tb isnt much to worry about though.
Another option:
make symlinks from your mount point to where ever you want.