I have recently succeeded in installing Endeavouros on a Raspberry Pi 4B. Fitting the desktop to a 1366x768 monitor was eventually solved by importing the edid.dat file from an old Manjaro OS, but I still cannot manage to achieve a typical KDE desktop. The system at present boots up to a screen with rows of very large clickable icons, no KDE-style panel, no response to right-clicks on the desktop area, and many application windows not having the top row with the window scaling and closing symbols, so needing Alt+F4 to close them. One exception to this is Firefox. Can anyone out there suggest where the problem lies and what to do to fix it.
Scaling factor in display settings issue? I had similar problem…
Are you using Plasma Login Manager instead of SDDM?
If so, at next login look in the lower-left corner of the login screen. I’ll bet it’s logging you in to GNOME on X instead of Plasma on Wayland. Just change it in the drop-down and login.
This drove me nuts the first time. Makes no sense that new installs of Plasma Login defaults to GNOME.
Strange…that was not my experience.
Did the offline install and Plasma Login Manager defaulted to Wayland and KDE.
A bug perhaps?
EnOS ARM images have installed plasma-login-managerfor KDE plasma as default for a while now. I doubt it has SDDM unless Original Poster manually changed it.
As @rubi1200 observed, I have never seen that on EnOS ARM with KDE Plasma before.
I run RPi 4b and RPi 5 on 1366x768 Samsung TVs all the time. So the only thing I can figure out is whatever brand of 1366x768 monitor @Bosquet has may not be compatible with RPi GPUs
EDIT:
I use 1368x768 on 32 inch Samsung TVs. This combination of resolution versus screen size produces a very nice sized text for my 75 year old eyes without messing with Scaling.
Pudge
I think we may be getting nearer an explanation. Having failed to get Plasma working with the correct screen size originally, I installed Endeavouros with MATE, and then installed Plasma and removed (as I thought) MATE. The initial boot-up screen has the KDE/Plasma identification at the bottom right. However, once I’ve entered my password and closed the Endeavouros Welcome window, the icon-infested screen shows up, but there’s nothing at the bottom left-hand corner to be clicked (not surprising, since although running the cursor around the edges of the screen clearly shows that the system thinks all four sides of the desktop are displayed, the bottom half of the second row of icons is not visible. So, (a) how do I ensure that all of MATE has been removed and (b) what bits of Plasma may be missing? By the way, Pudge, my eyes beat yours by 12 years!
@Bosquet I am taking my hat off to you! 
At last! brucew was almost right, but although I found nothing at the lower edge of the login screen to click, in my latest trial I took a closer look at the items along the TOP of the login screen. One of them was a drop-down with ‘Plasma bigscreen’ showing, which I had previously taken to show that I was safely in Plasma and didn’t need to do anything. HOWEVER, this time I opened the drop-down and found ‘Plasma Wayland’. The penny dropped, and with the latter selected, I was immediately back in familiar KDE territory with everything working in the usual way. I am now looking forward to commissioning my endeavouros installation to cover all the areas (e.g. e-mail, graphics, audio editing, data handling via a LAMP stack, etc, etc) that I formerly I had available on my manjaro system.
Many thanks to all who have tried to help an old stager reach this point. 