You might have sucess with avidemux - there’s no need to resort to the AUR (which hasn’t been updated since 2020) as both the command-line interface and GUI (qt) of AviDemux are available from the official Manjaro repositories.
sudo pacman -S avidemux-cli avidemux-qt
It’s not an overly complicated application to use, once some time is spent to learn it’s capabilities. That said, if you had difficulty with Handbrake, perhaps AviDemux is not for you either.
I mentioned much earlier;
It might yet be useful for you.
I have nothing more that might help with your k3b/transcode struggles, except to again mention that Handbrake works for me.
Regards.