Thank you for the responses! It is flowering well now so I'm guessing my expectations of how tall it would grow were what was wrong not the plants themselves
I've decided not to cut down the cover crop and just let the buckwheat reseed itself if it wants to. The people who lived here before me decided to use masses of landscape fabric in what I thought was a raised bed but now am thinking was just a 'picnic' area. I didn't know this when I planted into it but rather when I went to make 3 more beds I found the offending fabric. Instead of ripping it up I'm just going to use the area as a wood mulch composting zone with the exception of the 2 beds already in it. Those beds I'll plant herbs into-mostly annuals but also some mints that should do well in partial shade and hopefully be mostly contained by the fabric+wood chips all around them.