Gracias for your insight, @anon32327672!
You are really investigative (kudos!)
I finally figured out that using the screensaver was a perfect way to close one sesion and then start a new session in another console VT without passing thru horrible SDDM (yes, it’s light, however its documentation do not help to make a new theme, it lacks some features such as the one commented by you… etc.)
Going back to this question, I really want to understand LXQt way of handling X stuff. Mainly as you discovered previously, I want to know how to have another session (I use this technique when working on something personal and then switching to work stuff, and viceversa: each one of them with a different user).
Your suggestion is a nice command that I was not aware of. Thanks for that, too.
Since I have so little time with LXQt, I’m trying to understand it as it is with SDDM, then I will probably switch to LightDM.