This itinerary requires a visa. Transit without visa through Thailand is not possible when flying on low cost carriers such as Thai Smile. You must clear immigration in both directions to change flights, even without checked baggage.
Timatic, the database which airlines use to verify passenger travel documents, states in part:
Visa required, except for Passengers transiting through
Bangkok (BKK) with a confirmed onward ticket for a flight to a
third country within 12 hours. They must stay in the
international transit area of the airport and have documents
required for the next destination.
- This TWOV facility does not apply to passengers traveling
on Angkor Air (K6), Beijing Capital Airlines (JD), Business
Air (8B), Cebu Pacific (5J), Eaststar Jet (ZE), Golden
Myanmar Airlines (Y5), Indigo (6E), Jeju Air (7C), Jet Asia
Airways (JF), Jetstar (JQ), Jetstar Pacific (BL), Jin Air
(LJ), JuneYao Airlines (HO), Lao Central Airlines (LF),
Mandara Airlines (RI), Norwegian Air (DY), Okay Airways
(BK), Orient Thai Airlines (OX), Regent Air (RX), Scoot
(TR), Shandong Airlines (SC), Shanghai Airlines (FM),
Shenzhen Airlines (ZH), Siberia Airlines (S7), Sichuan
Airlines (3U), South East Asian Airlines (DG), Spice Jet
(SG), Thai Smile Airlines (WE), Tianjin Airlines (GS),
Trans Aero (UN), Turkmenistan Airlines (T5), T'way Airlines
(TW), Ural Airlines (U6) and VietJet Air (VJ).
You can obtain the e-visa in advance, to save time at Thai immigration.