My son started school early. He may not enjoy school but school isn't his life so he is learning to offset that negativity with the positivity in his life. If school made all kids that miserable to the point the couldn't ever bounce back then I wouldn't be ready to celebrate my ten year wedding anniversary with the most wonderful man I've ever met or advocating for my two happy healthy parent in a job I find stimulating and rewarding...so...I guess what I'm saying is that you are only as miserable as you allow yourself to be.
I think expecting our children to make a conscious choice to not be miserable when they are bored and bullied for a substantial part of their waking hours is a little unfair. This especially applies to kids with tendencies towards anxiety or depression or are 2e.
Since for some time in NZ early enrolment has been forbidden the only person I know who started before their fifth birthday is my stepmother. In her case it was a mistake because she was a perfectly average student who just happened to learn to read early because she had a slightly older brother and a mother who always wanted to be a teacher. For a gifted kid I would say staying out of school as long as possible may not e such a bad thing. I think I would prefer normal entry and skipping here because preschools don't do cademics. Other places early entry would be better.