The question is framing the situation well, and I can come up with two different possible solutions. One of them is shown below as I'll explain and the other one is a bit different version of the same solution, so that you may alternatively develop a version of the solution provided here.
Basically, you should indent
subItems in contrast to
and it makes dissociation with the rest of the content. You may choose
chevrons again with subItems (not in the same vertical line
but with indentation, horizontally aligning to the center of the rest
of the distance to the right) but using a different arrow-like glyphic
or using same chevron with a lower font-size might be your
Alternatively as a second solution to this is also having indentation with
subItems, but just includes separation points between
subItems. I removed them to make the degradation appealing but you may free to add it. But I'd add them a differentiation again to be more visible.