If I understood you correctly, you don't want to change the default behaviour of CAPS LOCK, but only the behaviour of the number row when CAPS LOCK's state is on. In this case you can have among others the following options.
a) Change type by block
ALPHABETIC (or #_LEVEL_ALPHABETIC if you wish to have more levels) for those keys which should ...
One option is to have a shortcut to always switch to first keyboard layout. For example, I have WIN+Space to switch to the next layout and LCTRL+LWIN to always switch to the first one.
How to pick that option depends on the program you use to pick the keyboard layouts and switching options. Here are 2 examples:
If it is localectl set-x11-keymap from command ...