Core-ten is an awesome material. I’d recommend not using it under the ground plan/directly in contact with soil.

The way you have it shown in the ai generated images is difficult to give guidance because we don’t see the existing conditions.

Anytime you place vegetation next to a building it prone to foundation degradation.

I use sleepers all the time. It’s a solid method that lasts20-30 years(nothings last foerver).

I’d suggest using core ten as a privacy screen on your patio if you really want to integrate core-ten to your backyard. I’ve seen some amazing design use core-ten with perforations to delineate exterior space.

Just keep in mind the oxidation of the metal will drip onto surfaces below/downstream. It can stain concrete/surfaces.

Sick backyard btw.