You mean - open the application launcher, right click a launcher and select “Add to desktop”? Works for me. This is probably a very specific crash - please report it in bugs.kde.org or using the Plasma crash reporting dialog.
Klassy is a fork of Breeze with a lot of extra options. It may be too much, or maybe you can use it for just what you want.
Or you can just fork Breeze yourself and have a “Breeze with less rounded corners” theme it shouldn’t be hard to just change the number in the source somewhere, and adding one more configuration input is not much harder.
I also tried that and cannot cause the crash. Either it was fixed already (I’m on Plasma 6.1.2) or you have some setting that triggers the crash - either way, reporting a crash is always a good idea.
I’m not sure what you mean by “global”, but Klassy is both a window decoration and application style so the corner radius applies to both.
Regarding Plastik - it’s a different theme so does things differently. You can add any feature that you want to anything that you have the source code for, but someone needs to do it and no one has done that for the old Plastik theme and I don’t expect any KDE dev to put work into that.
I’m not sure what you mean by icon files, but Klassy installs two icon themes: “klassy” and “klassy-dark” that contains icons that are used for for some buttons. Depending on how you installed Klassy, you will likely find them in ~/.local/share/icons or in /usr/share/icons.
ability to run bash scripts and commands from a notification (missing)
This was never promised for 6.1.
Kmail does not open contacts from search menu
KMail is not a part of Plasma.
cannot add shortcuts to desktop (plasmashell restarts and does not add the shortcut)
This is fixed now, in Frameworks 6.4. It was actually a Qt regression that we improved our code to fix.
a rounded edges input in the theme settings
This might happen in the future, but if so, it’s a long way off as it’s very niche and unimportant relative to the amount of work necessary to do it properly.