could you thin them out & perhaps put the thinned out ones in another container (larger one though so they could grow??)
It sounds to me as it they really need to be planted out, but as it is too early, perhaos giving them more room would buy them some time.
They are a nice size so they might be ok with transplanting some to another container.
I found the below on the internet. I hope it helps too.
usually become spindly when they don't receive enough light indoors. If you grow the transplants in a sunny window, ensure they receive at least six hours of direct light and rotate the pots daily so the plants receive sunlight on all sides and grow straight. growing
petunias 6 inches beneath a fluorescent light, left on for 12 hours a day, ensures the transplants receive sufficient light for strong growth.
Hopefully someone else will come by with an idea or 2.