I would wait until all die-off symptoms are completely gone for multiple weeks and your bowel movements are restored/normalized (most people should have 2 or more bowel movements per day). I would also try some of the stage 2 foods and even non-stage 2 foods to see where you are at.
For me, I couldn’t handle almond butter until 4 months on the diet. I used to get an allergic reaction but now I don’t. Same with regular butter. Consider these test foods. Not everyone has the allergic reaction either; I had real bad leaky gut which causes this.
As far as brown rice, I would maybe introduce it once a week at first and slowly work your way up. Buckwheat is a stage 1 food for most people is a rather safe vegetable.