I have three varieties in my garden; wooly thyme is lower-growing than the mother of thyme and lemon thyme.

Elfin thyme, mother of thyme and creeping thyme are all different types of thyme, elfin thyme (Thymus praecox 'Minus') being the lowest growing. Wooly thyme, Thymus pseudolanguinosus, is the next lowest growing. However these two very low thymes are seldom found available from seed: " Since cross pollination is a concern with thyme, it is very uncommon to
find seeds for low-growing varieties like Elfin Thyme or Wooly Thyme,
which will be propagated by cuttings to insure that offspring are true
to the desired characteristics of the varietal." That's why you only find these available by plugs or as started plants :-)

You might be happy with a thyme called 'Magic Carpet', which is a form of creeping thyme, Thymus serpyllum. It is a heavy bloomer in a bright magenta pink and stays low enough to be recommended as a filler between pavers. Seeds available here.

What a very helpful comment, both in terms of education and referral to a resource. Thank you so much!