With previous Plasma versions I was able to have 150% volume bar in Audio Volume Plasma Applet, but now it is only 100% even with Maximum Volume set to 150% from Audio Volume Settings.

I can do it.
https://i.imgur.com/sINvGKz.png

Operating System: Manjaro Linux
KDE Plasma Version: 5.17.4
KDE Frameworks Version: 5.64.0
Qt Version: 5.13.2
Kernel Version: 5.4.2-1-MANJARO
OS Type: 64-bit

I noticed you have reported other problems that I don't have. So they may be a distribution specific bugs or something wrong in your plasma cache or plasma configuration files. This may suggest that you installed plasma on top of another default for your distribution graphic environment. If yes, you had better install distribution that offer default Plasma, like Kubuntu, OpenSuse, Manjaro.