I wonder if there's a way to make Ubuntu templates that don't use
logical partitions?

The ubuntu 18.04 template was made without logical partitions. They are both in the default set of libguestfs templates.

# virt-filesystems --all --long -h -a ubuntu-18.04.img
Name     Type    VFS   Label  MBR  Size  Parent
/dev/sda1  filesystem  ext4  -    -    10G   -
/dev/sda1  partition   -     -    83   10G   /dev/sda
/dev/sda   device    -     -    -    10G   -

P.S Sorry I couldn’t figure out how to reply to the existing thread.