I am not a computer scientist, you may consider that the problems exposed should be divided into several categories for specialists.

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My setup:
Distribution Debian 11 Bulseye
Version of KDE Plasma: 5.20.5
KDE Framework version: 5.78.0
QT version: 8.15.2
Core version: 5.10.0-10-amd64

Processor: 4xintel core I5-6400 CPU 2.70 GHz
Physical memory: 7.69 Gio
Exchange memory: 15.26 Gio
Graphics Processor: mesa Intel HD Graphics 530

HD 1: ST1000DM003-1ER162
BUS: SATA

HD 2: ST1000DM003-1SB102
BUS: SATA

Écran : 3840X2160 px

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I recently installed Debian10 Buster and the version of KDE Plasma which is proposed with (see above).

I discover that this version of KDE is less customizable than the previous ones.
1) In particular, having a screen of 3840x2160 or resolution about 125 px/inches, I need to adjust the width of the scroll bars. The default width is too thin and I have trouble reaching the cursor. The current width, measured on a screenshot, is 5px. I would like to move to 15 px.

2) The KDM window or what acts as a start screen is written in very small as if configuration settings were not taken into account. For the record, I set the Noto fonts without 14 points for my 4K screen. For the record, 14 points is a bit small for me, but at 16 points the display is incomplete or too overlapping with the top line.
Also, in this window, the password input width is a little fair to be able to track if the keys are properly taken into account. My password is 13 characters. I found that sometimes the seizure of a character resulted in 3 to 5 points! (involuntary repetition) That's why I think it's necessary to be able to visualize the feedback on the keys.

3) In the system configuration panel (systemsettings5), I can't find where to install the backup screens.
I tried a separate installation, but the demon takes too many resources and ends up slowing down the whole system. In the past, this used to be found either in "Workspace Behavior" or in "Energy Management. For my part, this function being closer to energy management than to normal workspace, it seems to me more logical to manage the screen savers in this section. Especially since the different waking times are in this section.

So here are three concerns that should concern other users and that I would like to see resolved in a future update.

Cassis

It is one of the reasons I don't do Debian in general anymore. Even well over a decade of openbox only , I moved away from debian altogether. It's a known issue the Debian squad doesn't give a d.... about. It's the scaling issue. It doesn't matter if it's gtk or qt, it will always look cramped. At a first glance it's very noticeable in a list view. The margins are cramped ( by lack of a better word). There used to be a workaround by adjusting the gtkrc, but that no longer works. Now, if you're on some gtk setup, you could say "it's do-able". But imho, a kde environment becomes problematic. I've tried several times with regular debian ( stable, sid), siduction, MX kde, Sparky and some....to no avail. I fiddled with fonts, scaling and what not...no use. Maybe someone else has the right answer to this but I'm willing to bet that , even if you could get some scaling right, using non default stuff like, say certain kwin scripts, krohnkite, parachute.....well, good luck with that. To this day, I haven't found any reasonable solution that does the trick....for the Entire desktop. So, I'm afraid I can't help you. Imho, kde is okay...just not on debian.

I found some screenshot I posted a while back. They are pcmanfm screenshots, both in debian and an ubuntu based one. Exact same size of windows. Now look at the selected item in the sidebar.
The one in the back is the Debian based one. Again, exact same size of windows. See the difference in selection height? The toolbar icons are the same size though. But the overall ui is..off. You can already tell by the height of the toolbar difference. I use corner bindings on openbox, one of which is assigned to skippy-xd. Even that is looking...off. If you'd try parachute in debian, I'd say the size difference is about 10%. That's something you'll find allover debian, cramping.
https://imgur.com/a/9uI03UY

For information, I have taken some steps hoping to document the developers.
The letter heights are taken from the base line to the top of the shaft. I checked that from the top of the eye to the bottom of the jamb, it was the same dimension.

The height of the title letters is 23 px for the three windows
The height of the main menu letters is 23 px for the three windows
The height of the side menu letters
Dolphin: 23 px
Writer: 20 px
Gimp: 19 px

Scroll bar thickness
Dolphin: 11 px
Writer : 10 px
Gimp : 6 px

Size of the icons
Dolphin: under the menu bar 98 px
Writer : under the menu bar 45 px
Gimp : under the menu bar on the left (toolbox) 36 px

I remember that a few years ago, the font harmonization was much better and you could adjust the thickness of the scrollbars. Was this due to the fact that the display variations were smaller than today, I don't have the answer.

In my kubuntu 18.04, there was the possibility to adjust the size of GTK fonts, but there was no longer the adjustment of the thickness of the scrollbars.
Nowadays, 4K screens are becoming more common and the different resolutions of these screens will cause problems for KDE if the developers don't provide appropriate adjustments for the users. We have to take into account that not everyone has the same eyesight, nor the same dexterity.

Hoping to contribute to improve this desktop that I love more than all the others.
Cassis

If you know how, you could install classik ( application style/window decoration). You'll find it on github https://github.com/paulmcauley/classik. That one does have, amongst a whole bunch of things, scrollsize adjustment. https://imgur.com/a/FkJ1wA2 Remember the scale issue in debian though. Also, it's possible you'll have to do some adjustments since it's, well, debian, you're running. By that I mean the plasma version.

It'll look like this, with the new theming style. https://imgur.com/a/9L9xRsh

In the READMEmd file of Classik,
https://github.com/paulmcauley/classik#readme
it is specified in the
Installation
ClassiK from version 3.0 onwards requires a recent version of KDE Plasma with at least KDE Frameworks 5.86, so please check you have this in kinfocenter before trying to install. You can either download the pre-built packages or compile from source.

I only have the Framework 5.78.0

Thank you for trying to help me.
Cassis

Well, unless you do a different distro, I'm afraid.... Even on this 5.22 ( framework 5.86), the config itself wasn't installed in the system settings window. ( I merely needed the window decoration part though and made a menu entry for the config tool, so, no prob.). I checked the Lightly application for scrollbar settings, but no cigar. Maybe there's another application style out there that can handle sizes but... For now I'm afraid I can't help you. Apart une loupe

I suppose you could edit the scrollbar through and slider size in /usr/share/themes/Breeze/gtk-3.0/gtk.css. Would I recommend it? Nope.
Wait. Doesn't Oxygen have such a setting?

Always in the spirit of enlightening developers and users, here's some data collected on my 4K display.

In System Setup → Hardware → Display, there is: Overall Scale.

I made it a habit to open Writer and display a blank page.
With Writer's Zoom set to 100%, I'm looking for when the global scale gives me a display close to the width of a sheet of paper on the screen.

I got this result with: Global Scale = 168.75x
(I noticed that the scale increment was no longer free but had a step of 6.25x).

On the other hand, I made a file of 1200px long by 250px high which is used to check the indications of the kruler utility.
In this file, I made a scale indicating the pixels with marks every 5 and 10px
A screenshot with Kruler cut around the ruler is copied in my test file of 1200px
There, I notice that for 100px displayed by Kruler, I count 167px
and that for 600px displayed by Kruler, I count 1 011px, which is consistent since it corresponds to coefficients of 1.67 and 1.685, which includes the margin of error of appreciation of the graduations.

Here is my small contribution.