I think the problems are manifold here

started growing too hot, it's been in the 90s all summer

put too many in the same container because I'm not Container Rich

soil probably isn't nitrogen rich because I errrr uhhhh have been re-using soil, but I do occasionally give it a bit of that Miracle Grow (tm)

the most Radishy Radish I'm growing has been growing for probably two months now, and I destroyed all the others in its container. It's still not getting much thicker though. Every single one of them seems to be growing root tissue above the soil which I feel like might contribute to them not thickening up

I figure I'll just keep watering them until they either seed or die