You don’t need to go into the
blivet-gui interface. When the installer detects the partitioning layout, it asks you to choose the mount points and mark the partitions you want to reformat. So choose your mount points normally, but do not format
/home (it should show up as the label you have given it:
homeship), but choose to reformat the
/ partition (as per your label:
rootship). You are making it more complicated than it actually is.
All this information is presented in the installer UI, if you read carefully, you won’t be lost. If some of the wording is unclear, then definitely you should ask here (preferably with screenshots).
EDIT: just to clarify, you need to go to the
blivet-gui interface, iff you want to repartition your disk, but IIUC you are not, you just want to reformat it.
PS: If you feel more comfortable doing the reformatting yourself, then you could do it manually before you start the installer, and during installation just select the mount points and proceed.