I've had a quick go and I must say I like it a lot!  Great job, Mathieu!

A few things:

- When booting on my desktop with a RX580 GPU, I end up with a blank screen.
Turns out that only the display is somehow failing (still investigating), but
pressing Ctrl-Alt-F2 brings me to the documentation!

I suggest we add a 10 second-timeout to the graphical installer so that if there
is no user interaction, it automatically switches back to TTY3 with an
explanatory message, e.g.

--8<---------------cut here---------------start------------->8---
There was no user activity on the graphical installer which may be due to a
display issue.  You can proceed with the installation via command line from
here.
Press Ctrl-Alt-F2 to go to the documentation screen.
Press Ctrl-Alt-F1 to go back to the graphical installer screen.
--8<---------------cut here---------------end--------------->8---

- Add and extra warning before partitioning!  I did it wrong at first but I did
  not expect the installer to proceed straight away, I thought I would have a
  second chance at reviewing the changes before proceeding.

- More generally, I like the idea of a two-stage process: first collect the
  details from the user (without actually performing any change), then show a
  summary of the changes, ask for confirmation and only then apply all the
  changes in one go.

Thanks again for the great contribution!

Cheers!

-- 
Pierre Neidhardt
https://ambrevar.xyz/