How about we swap out planes for strings.
[Disclaimer - I don't know anything about actual string theory. I'm just stealing a term because it sounds cool and kind of fits this conception of planes and dimensions in fantasy games]
No, the elemental plane of fire won't look like a tightrope - it will still be an endless reality of fire and efreet. But it will functionally be a string, as will all the other planes. Dozens of planes, as strings, crisscrossing the multiverse and, at certain points, touching.
At a planar nexus, the reality of two planes (or more) are combined, forming a little pocket dimension (perhaps a planet) that combines the aspects of the planes that are connected. So you might have a plane that combines Elemental Fire with Chaotic Good, or a plane that combines Shadow with Water. Maybe the Astral Plane is where all these strings are located - and maybe when traversing that plane you can accidentally hit a string and find yourself in another plane. The Ethereal is just the blurry edges of the vibrating Material Plane, and can be a passage to these "nexus points" that interact with the Material Plane.
Maybe the Material Plane is just a nexus point between all the different planes of reality?