Went with a mate to Tukcom Pattaya.
He got an AIS SIM with 15 Mbit/s (fully sufficient for usual mobile use, messenger, social media, youtube, FB, insta etc.).
Cost 450 Baht for 30 days if I remember right all incl.
Tons of other options from DTAC with speeds from 10 to 30 Mbit/s WITHOUT volume limit.
(AIS usually has)
Prices about 150 B to 300 B for 30 days (plus cost for SIM and service/registration).
Be aware that the 30 days start with the registration (if you would do in your name ahead of arrival).
Best would be wait until they are here.
About every mobile phone booth can help.