You could do a castsequence but they can be unreliable.
The reason your original macro works with one it because it’s going… if you’re not in bear form then cast bear form else cast thorns. There’s no test to check “if the thorns buff is up”
This is the castsequence version
#showtooltip
/use [mod:ctrl] 13
/castsequence [noform:1] Bear Form(Shapeshift); Thorns, Frenzied Regeneration