I only change pots when they're already distressed (bad mix etc) or when the roots are at the bottom of that pot. All the ones I moved to buckets had the root already at the bottom of the 100mm seedling pot and were on the verge of being root bound.
I had to pull and discard about 15 plants yesterday to make room for healthier ones. I potted up most of the seedlings left that showed positive growth. Most of the sick ones hadn't spread their roots into the buckets at all. Although some had but were sick anyway. The sickest ones had wet soil in their buckets. I'm pretty sure the problem was a combination of poor drainage due to the soil mix used and the plants being too small for the large sized bucket.
I've changed the soil mix to just budget potting soil, a lot of perlite and no nutrients included so I can control what nutes they are getting. Didn't use any coir-peat because retaining moisture is the problem at the moment. When it gets really hot, and if the soil is repelling water I might use a soil wetter. The roots of the new ones had filled their pots and I had to tease them out a bit so I'm confident they will fare better.