these were sown in february, grown very slowly to this size and stopped, the leaves look very healthy, anyone grown them from seed, how did they go ?
I wonder if perhaps you're being too nice to them. They grow like weeds in our garden and self-seed freely. Their favourite places are jammed together in hot dry areas like cracks in concrete and between slabs. When I grow them from seed, I keep them crowded and on the dry side in good light with multiple seeds in pots and I don't prick out until they're fighting their way out. Then I pot on into individual pots that are a size smaller than they need.
My Agapanthus seedlings sown earlier this year are about the same size as yours - I have mine in small pots and keep them crowded and then pot on. Be Warned - they take about four or five years to flowering!
We lose a good 50 percent of seedlings over winter, even under cover. Outdoors we lose them all, even establsihed ones, even planted at the base of a south facing wall in well drained soil. The seedlings from about 4 years ago are just beginning to flower in the frames.