How to set Caps or Alt_R to input-layout-switch'er?

meinherz

I was trying setxkbmap
setxkbmap -layout "mars,moon" -option "grp:(caps_lock_)toggle"
it does not work.
And in dconf, I tried all the google recipes. I'm getting close to being disappointed. Is it even possible, maybe I'm wasting my time looking for a solution?

Budgie 4.2

meinherz

As a temporary solution, I chose the "Menu" key to switch the layout. It's a pity that no one knows how to set up switching to "Caps Lock" or "Right Alt" in Budgie.