Hi @TKG,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
Generally, in landscaping applications, you would use what is referred to as bugle/batten screws. The head type isn't super important, as it is just a method of rotating the screw, but from my experience, hex head or torx head screws will slip far less than phillips head screws do. Bugle/batten screws will usually be hex or torx head screws, and packs often contain a driver bit that corresponds to the screw head.
With treated timber, it is best to use coated screws, as the treatments used in sleepers can react with the galvanisation on screws if they are in direct contact with each other.
In saying this, I would typically recommend using longer screws with sleepers. If you are going through 50mm of sleeper, with a 75mm screw, you are only getting 25mm of embedment. This is not going to be particularly strong, especially when you consider that a garden bed has constant outward pressure applied to it from the soil and water it is holding. Instead, I would suggest using 100mm screws like these Bremick XTP Construction Screw 18g x 100mm. With 50mm of embedment per screw, you will have a lot more holding force than with the 75mm screws.
Let me know what you think, and if you have further questions, please don't hesitate to ask.
Jacob
