Installed twice before - once with User as admin and root - without automatic login and arrested at tty and installed again User with Sudo as Root

The normal Debian/Devuan way is to establish a root account, and a normal user account. Assign a root password during installation different from the user password. Then you work normally as user, and do administrative stuff as root in a root terminal. This way you do not need this sudo stuff.

Even with a cli installation without any desktop, the command line tools for adding/modyfing users and groups will be present (see Delgados response). Downside: you need to log-in in a root terminal and do the work by typing.

Depending on the desktop you will find one or the other graphical tool, e.g. Cinnamon brings a limited user/group graphical tool.
Gnome-system-tools is the more convienent tool, I also add this tool to my installations sort of at the end of the fine tuning.

BTW: For a multiuser-installation I would go away from slim (which isn't maintained any more anyhow) and use lightdm instead. Or sddm for KDE, which isn't my personal preference.

The desktop permissions are typically set by policykit. Each desktop should come with some set of rules to allow for user access to removable drives in combination with group permissions.

This is a really really really bad idea. Don't do it again. Never ever. As said a thousand times before.

For starters:

Just install and use it!
There is no need to do something special. Do not repair, when it's not broken!

lxqt uses the Qt librarys like kde, but it is neither kde nor gnome.
Anyone one can write nonsense in forums - including me of course.
Search: try "adduser", "usermod", "chown", "chgrp" . Adding "linux" or "debian" may improve results. A simple sudo adduser fred is usually sufficient.

Doku:
"devuan" is "debian" without systemd.
General linux: The archlinux wiki is a good source (wiki.archlinux.org), also gentoo.
Search results on "stackexchange" are often helpful for specific questions (it has several sites with different names to sort topics, but the same visual style).

Change DM on debian (e.g.):
sudo update-alternatives --config x-window-manager or
sudo dpkg-reconfigure slim  or sddm, depending on you preferences.

Maybe re-set or re-install lxqt ?
user:
delete or move lxqt-stuff in "$HOME/.config/". Subdir "lxqt", but maybe there is more.
system:
sudo apt purge task-lxqt-desktop
sudo apt install task-lxqt-desktop --install-recommends

I have failed to set up users and groups so that they all work and i can access internal data drives because LxQt seems confused as to whether its KDE or Gnome.  IN either install i have NO ability control users and groups. And it is not recognizing my data drives which have ALWAYS been instantly recognized  and accessible no matter who is logged in, what desktop.

Installed twice before - once with User as admin and root - without automatic login and arrested at tty
and installed again User with Sudo as Root

Neither time have I been able to control change add etc involving 2nd and 3rd users ( which i HAD intended to use separately for each desktop ).

I am going to reinstall again, this time installing Gnome-System-Tools BEFORE i try to add Users and groups.  But why should i have to find out by failure that Gnome-System-tools  is required BEFORE you create any additional users or groups?

at least thats' what i was told in a 3rd-paraty forum.

Sure home Daedalus gets its act together on this count.  Same problem, same desktops and a confused sddm slim x

openrc works well, i must say..

I don't expect a lot of attention here.   Just tell me the 5 or 6 or 7 things  that one should do after install but before doing anything else.  Thanks guys.

Meanwhile i will continue to look for answers in the forum but have had no luck to date.