Resize your partition to free up some space for a new partition. You will have to shrink the filesystem and then the partition. Install Linux on the new partition. Once installed, you can mount the old drive and set it up to mount as /home
Then clean things up a bit.
Assuming you used LUKS, you will have to know your passphrase to mount the encrypted drive.
Once you have a /home separate from your boot partition, it is a lot easier to distro-hop.
Resizing is a bit risky though. If you can afford even a small SSD, installing to that and then mounting /home is safer.
