How to move my files/directories from my boot m2 to a raid and or other ssd?

I am running Endeavour ver. 8/27 Os on a custom build. The motherboard is an Asus z97-a. with a Kingston 250Gb nvme m-2 to boot from. I sort of understand the /home with everything stored there. I have created a raid with samsung ssds and have a Seagate 2Tb ssd also.

My plan was to keep my Os on the Kingston and as little else as possible. I would like to keep my applications on the Seagate and my music and work in progress on the raid. I do graphics, audio mixing and 3d animation. Is what I am suggesting practical and/or possible?
Thank you in advance for any advice you may offer.

I’ve done the separate home partition before but IMO moving other things may be problematic.
The trick would be to retain enough “files” to get to the stage where /etc/fstab is processed.
After that all files would be accessible.
I had a look at / and in my case it looks like this (editted - for simplicity)

$ ls -l /
total 84
lrwxrwxrwx   1 root  root     7 Nov 11 09:14 bin -> usr/bin
drwxr-xr-x   5 root  root  4096 Dec  2 07:48 boot
drwxr-xr-x  20 root  root  4140 Dec  4 11:40 dev
drwxr-xr-x 125 root  root 12288 Dec  4 16:42 etc
drwxr-xr-x  12 root  root  4096 Mar 21  2020 home
lrwxrwxrwx   1 root  root     7 Nov 11 09:14 lib -> usr/lib
lrwxrwxrwx   1 root  root     7 Nov 11 09:14 lib64 -> usr/lib
drwx------   2 root  root 16384 Jan 14  2020 lost+found
drwxr-xr-x   2 root  root     0 Dec  4 09:30 misc
drwxr-xr-x   2 root  root  4096 Nov 13  2019 mnt
drwxr-xr-x   7 root  root  4096 May  2  2021 opt
dr-xr-xr-x 530 root  root     0 Dec  4 09:30 proc
drwxr-x---  16 root  root  4096 Nov 14 10:43 root
drwxr-xr-x  37 root  root   980 Dec  4 09:32 run
lrwxrwxrwx   1 root  root     7 Nov 11 09:14 sbin -> usr/bin
drwxr-xr-x   4 root  root  4096 Jan 14  2020 srv
dr-xr-xr-x  13 root  root     0 Dec  4 09:30 sys
drwxrwxrwt  14 root  root   520 Dec  4 17:31 tmp
drwxr-xr-x   9 root  root  4096 Dec  2 21:05 usr
drwxr-xr-x  13 root  root  4096 Dec  3 07:06 var

Ignoring /home the largest sub-directories are /usr and /var see below

$ sudo du -sh /*
0       /bin
118M    /boot
380K    /dev
15M     /etc
44G     /home
0       /lib
0       /lib64
16K     /lost+found
0       /misc
4.0K    /mnt
687M    /opt
0       /proc
29M     /root
1.7M    /run
0       /sbin
48K     /sd
12K     /srv
0       /sys
7.8M    /tmp
18G     /usr
23G     /var

Running qdirstat shows me that /var/cache is 17.4 Gb
There may be ways to reduce the cache size - (if that matters) - I don’t know.
FWIW I use 50 - 100 Gb partitions ( I have multiple distros installed ).
With a separate /home partition 50-60Gb for root would suffice and more robust.
but I would use a 100Gb root partition just to be on the safe side.

Thank you for your advice. I’ll give it a go and see what happens. Still not sure how to deal with the raid. Is the whole raid just another /usr or /var under the subdirectory /home?

Not sure I understand the question about raid …
If I had a raid I would use it to store data that is “important”.
It does depend on what you view as being important - very much a personal choice.
I wouldn’t view /var as being important, certainly not the cache dir.