Currently there are 2 different patterns of adding links:
When you are at the top level, to add a sub nav you can only click the sub nav link. However, when you are in the sub nav a '+' icon shows.

The confusing point is that of course, you can add a sub to the 2nd level item, making a 3rd level menu. However, this reads a little confusing.
The entire thing can get really complicated to navigate visually if you see the top level:

I would suggest we need to only show 2 clear ways of adding things by default and look at how we can focus depending on where you are.
This also gets worse the more sub nav you have:

First time commenting so hope I didn't miss information from other discussions.
I agree many sub nav "+" together is confusing. How about not having the sub nav "+" at all? I saw the sub nav "+" only shows up after the second level, which means when users start using the sub nav "+", they also know how to use the sub nav link.
Having only one way to add sub nav also makes "+" icon less ambiguous - "+" is for new link and sub nav icon is for sub nav.
Maybe we take a cue from the Block Navigator UI and add some structural indicators for sub-navs:

I like that @shaunandrews, let's explore this in a PR and then see what it feels like. Removing design feedback to progress.
let's explore this in a PR and then see what it feels like
@karmatosed @shaunandrews Just noting that there might be some overlap between this and https://github.com/WordPress/gutenberg/issues/18310. Do we need to take account of that and/or update the designs?
There absolutely is overlap, I think we can close out this issue and focus on the menu looking like the one that we have in #18310
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Maybe we take a cue from the Block Navigator UI and add some structural indicators for sub-navs: