Why does the Ubuntu installer not offer a way to create a custom partition table for an encrypted LVM installation?
I wish to have at least /, /home, and data partitions and a swap. I used to also have /boot.
I looked a lot but not a single post or webpage provides a good answer. This one offers a guided solution (not verified) but does not mention encryption.
You are using the wrong installer
The normal Ubuntu Desktop installer is intended for use by folks who have never heard of LVM, and have no opinion on encryption.
So the choices offered are limited for experts like you.
Ubuntu offers several ways to customize your install in advanced ways. The easiest is the Alternate Installer. Don't be confused by the phrase '-server' in it; simply install your preferred desktop environment on top when your customization is complete.