I know that this is the obvious setup for most people, but you won’t be able to configure this while logged in to the desktop environment right? Because tons of the DE configuration are stored directly in /home.
Boot to the live media, copy all of the files from the existing /home/username to the new volume /mnt/username (rsync -av works well for this) then alter /etc/fstab to mount the new volume at /home and reboot/
Yes, you would need to mount both the old /home and the new volume at appropriate locations but it would be quick and easy.
Maybe create /mnt/home and mount the old /home there, then create /mnt/newvol and mount the new volume there.
A simple rsync -av /mnt/home/* /mnt/newhome/ should work very well to move the data and keep everything within the users home directory on one volume.
Maybe for those who are critical for speedy access to configs and caches, or for a server environment, but my /home resides on an mdadm raid 5 array. The entire OS with all the apps resides on an SSD and I cannot tell there is any performance impact to having /home on HDD raid.
Configs are read once and forget when the app is started, cache is frequent for some apps such as browsers but occurs in the background so is insignificant to performance.
The only time I am aware of where differing read/write speeds would have a significant impact would be if you were using the different devices in a single file system, such as spanning a btrfs or lvm volume across both an SSD and an HDD so writes may span both devices.