Even when adjusting the mouse pointer acceleration properties (via KDE
Control Center > Peripherals > Mouse > Advanced tab) to 1.0x, the
mouse is still way too fast. Any suggestions on how to slow down the
mouse to a usable speed?
My experience was that if I set it to a slower speed, that worked.
If I then logged out, and logged back in (or rebooted), it was at the original higher speed. If I went to the settings page, that indicated the lower speed. Apparently, that speed had not been applied. I had to change something on that settings page, then change it back, so that the “apply” button was not greyed out.
I decided to live with the higher speed, rather than have to do that for every login.
What mouse model do you have? If you have a laser mouse, use a dark (ideally a black) mouse pad since that makes the mouse slower. However, bright underground and in particular white makes the mouse faster.