Hemp is a seed, but it’s one that I haven’t gotten allergic to despite consuming a pretty big amount of it. Most natural grocery stores have bulk bins that sell hemp seeds – you could try just purchasing a handful and testing a small amount to see what happens. Have you tried chia seeds? They also have protein and you might be able to tolerate them.
There are vegetables with protein too. Kale is the best one that comes to mind, plus things like spirulina (which you can take it pill form if you think it tastes bad).
I think oat bran might actually have more protein than buckwheat. Have you tried it?