Here is a closer photo of the elevated raised bed.

One really cheap option for such a large/tall container that may make up for some of the frustration is to grow potatoes in straw. You start them at the bottom and then occasionally add 4-6 inches of straw as they continue to grow through the season.

Here is a link that might be useful: Potatoes in Straw

I have four raised beds that I use for SFG. They are on their 4th year. I gave up on trying to find the ingredients for Mel's Mix the first year and just filled them with a mix of garden soil, mushroom compost, and some peat. The soil level drops several inches each year and I have just been topping off with compost and maybe a bit of soil. I've had excellent results from this. Did you thoroughly wet/mix your Mel's Mix before putting it in the box? I know that's a common downfall when using any peat-heavy mix.

If it's not directly touching dirt on the bottom (IE, dirt-to-dirt), it's not a raised bed, it's a container. And Mel's mix isn't particularly suitable for container gardening, it just doesn't drain well enough.

The dirt-to-dirt connection basically acts as a wick, and pulls moisture out of the bed, so Mel's mix is geared to hold a lot of moisture.

Typical container mediums are bark, peat, and perlite... much faster draining than compost, peat and vermiculite.

Replacing all the soil is probably a non-starter at this point, so I'd suggest putting some wicks in place to pull some moisture out. You can probably read about wicks in the container forum.

I don't eat white potatoes and this is my only garden spot. I bought it to keep my dogs out of it! I agree that is essentially a giant container.

I am just gonna rock potting soil and jazz it up next year with compost etc.

Will keep y'all posted!