I still have no idea what "does not have any text in it's interface" means. If you look at a screenshot, there's no text on the interface until you open a program/menu. See https://lxqt-project.org/
And though I don't think it's the problem, if you're using sddm the "exec.." part is unnecessary. It might be, it could be trying to start lxqt twice.
> I try other lighter desktop managers but: - most of them does not have "settings" GUI application
Have you already had a look at LXQt? It worked pretty well for me and with lxqt-config it also has a graphical interface for the settings. Not even 24 hours ago, a new version of LXQt was released, so the project is still active.
>But KDE is not light
Depends on the definition, I would say. On my computers with KDE Plasma, approx. 550 MB of the RAM are occupied directly after booting (incl. systemd, pipewire etc.). That may be a lot compared to other alternatives. However, if you take into account the functions offered by KDE Plasma, I think this is a relatively small amount of memory. Apart from that, unused RAM is wasted RAM. In my opinion, this also applies to other resources as long as it does not cause any problems. Because less resources used does not necessarily mean that something is faster.