If you aren't concerned that the flat panel doors will look "cheap", then don't be concerned that the island back will look cheap. The two need to match, or it will look incoherently jumbled. A better solution is not a sheet goods panel, but a wainscoting panel composed of slab doors, just like you have chosen for the fronts.

Thank you! That's what I was thinking originally but then I get inside my head and second guess everything

I’m not quite clear on your question but I can say that my larger island in my kitchen is backed with wood beadboard - ebony stain in color - same as the cabinets on the kitchen side of the island & the hood over my stove. My perimeter cabs are a cinamon color stain.The cabs aren’t beadboard doors. So 2 different surface styles but matching color to front of island. It’s great having the wood back to island & the darker color too because if it were the usual wall boad on island back & painted same lighter color as kitchen walls there would be many scuffs from feet hitting the island wall while sitting there. I’m not sure I’d like a surface that looks like cabinet doors & doesn’t match the cabinet style in kitchen. Not knowing exactly what your cab doors look like, maybe take a look at beadboard. It’s pretty versatile & the lines / texture in the bead board would break up the white surface as well as hide scuffs a bit more.

I'd explore option other than a long flat panel. Painted bead board is one. White Oak is another with the two corners to match other corners in the cabinetry. White slab doors without handles is another choice.

You can also add interest by using panels in a curved faceted arc to create a seating layout where people can see and talk to each other. By making the base faceted, the top can be faceted or curved. This is only for the seating side of the island.

Imo, the rectangular island will be the defining passe kitchen element of this era.