I'm in a similar situation - I am flying Thai into and out of BKK, but there are segments in my itinerary served by Thai Smile (e.g., Phuket-Chiang Mai).

I am Star Gold and am checking in 1 suitcase for these short-hauls, but don't want to incur any fees on Thai Smile. From what I've read, it should be fine, but hopefully the experts can confirm. Thanks!