I went through almost same steps, just to change to size of the default /boot partition. I gave up and modified the parameters of the default installer. If you are also only trying to have a separate /home partition, take a look at this: https://askubuntu.com/a/678074/313386.
You can change the default recipe to include a /home partition. Boot into the live CD and take a look at /lib/partman/recipes-amd64-efi/50home. After saving your changes, you can tick LVM and encryption options and select "Use the entire disk".