i not like the sub floats because you can not start direct, they have no visible state after deploy.
the user wonder why the lines are dotted.
its better so make a segment (user node) with a entry and exit point that can be folded optional at the same flow page.
maybe take a look at construct 2 or unreal engine for inspiration, they using visual design too.
i believe my wish is some kind of node in node. i am missing also a in/out (start/end) at the flow itself.
as developer i thinking in functions blocks or tasks.
they can have much details at design but for usage its more a black box.
the designer seems to use a fix design area but it should be auto size based on contend.