There is no option for making USB installs in Calamares.
Your only option to get it right is a manual installation. Albeit not being about KDE and including encryption - the following topic covers how to install Manjaro to USB
To answer your question directly could be bad if the context is different and you are the only one knowing the exact setup.
The above guide contains information on bootloader and what to change. Scroll down to Install grub.
Also take the time to check the USB specific consideration section