I don't see how you could make bonsai of Thyme. It has cute little leaves but is basically just a creeping groundcover. It might look nice planted over the soil surface in a bonsai pot, like moss, but not as a bonsai itself. I have some that's been growing outdoors for several years (it's hardy to zone 4) and it never grows taller than about 2-3 inches. Rosemary on the other hand, does develop a woody trunk and I've seen it trained as bonsai. I have a couple in pots that I've been trying to grow trunks on.