Q.leggy seedingss
when seedlings get too tall, is it possible to put them in a deeper hole, support them and then gradually fill in more soil?

Certified GKH Gardening Expert
That would probably lead to stem rot and a fungal disease known as dampening off. The best course of action is to increase the light source and water less. Seedlings become leggy from lack of light and too much moisture . This article may help
https://www.gardeningknowhow.com/garden-how-to/propagation/seeds/leggy-seedling.htm