I go to Hatyai every month by bus (RM60 - RM80 one-way)  from TBS (10pm/11.59pm). I find the bus timing is better and more convenient than taking the train. The bus is direct from TBS to Hatyai town and usually I'll reach there around 8am, drop my luggage at the hostel then go breakfast and jln² but if by train you'll need to take another train or minivan at Padang Besar and reach Hatyai town at noon.
For accommodation I always stay in hostel (THB300/night), but if you prefer private room there's plenty of decent one between THB1k - THB1.5k. Foods is rather cheap, usually i spent like THB50 - THB100/meal.
For transport I would suggest you to book Grab or InDrive, they got motor options there if you are not renting a bike. Sometimes I'll use the songtheaw (small truck bus) too, it's cheap (Central to Lee Gardens area THB20)  but have to wait a bit.
You can buy your return bus ticket at the travel agencies around the town area for THB700 (used to be THB600 before we removed the diesel subsidy). Take the 10am bus and you'll reach TBS around 8.30pm.
PM if you need suggestions for makan, minum, dispensaries, massage and boom² 🤭