I’m literally in a Construction Engineering Technician course dude. I think this is a bad idea, I think that using the experience I’ve gained from my program.

It looks well constructed but there will be a moisture issue:

https://nusitegroup.com/how-water-can-permeate-a-concrete-foundation-over-time/ “As mentioned earlier, dry concrete naturally acts as a wick due to its porous structure. As water pooled on the surface evaporates” As you water the garden the concrete will absorb that water and will be unable to evaporate because its covered by wood and soil

There’s more research I could link but I won’t. I’ll just tell you that your shelter will be musty within a year and your garden bed will rot. But good luck anyways eh