Keyd is a good alternative for KMonad. Less advanced / less configurable but easier to set up, while still offering layers.
I haven’t used keyd before, but it looks like you could do a simple momentary layer like this
# R-Ctrl + arrows = alt arrow functions
[ids]
*
[main]
rightcontrol = layer(altarrows)
[altarrows]
up = pageup
down = pagedown
left = home
right = end
Looks like you can even do a momentary plus locking layer like so
# R-Ctrl + arrows = alt arrow functions
# R-Ctrl + / = toggle lock of arrow alt functions
[ids]
*
[main]
rightcontrol = layer(altarrows)
[altarrows]
up = pageup
down = pagedown
left = home
right = end
/ = toggle(altarrowslock)
[altarrowslock]
up = pageup
down = pagedown
left = home
right = end
I don’t have keyd installed, so this is untested.
~edit~ config error in momentary plus locking corrected