Hi @ali86,
Welcome to the Bunnings Workshop community, it is wonderful to have you with us.
There would be two main ways to approach this project: the way you have mentioned, which uses sleepers and fibre cement sheets, which would be relatively simple to achieve D.I.Y., and the second, more durable option is using besser blocks with render applied to them.
Your proposed approach is a reasonable way to achieve a rendered look without the cost and labour of full masonry, and it does not automatically compromise quality if it is detailed properly. Using sleepers for the structure and fibre cement sheet as a substrate for render works because the timber frame carries the load, and the fibre cement is only acting as a cladding. From a DIY perspective, this keeps the build simple and accessible, and from a cost point of view, it is likely going to be cheaper than blockwork.
Durability largely comes down to how you manage moisture and movement. Timber will naturally move due to expansion and contraction, and that movement can transfer through to the fibre cement and finish. You can limit the wear by lining the inside of the bed with builder's plastic so that moisture doesn't transfer through the timber, and allowing for drainage at the base. On the outside, a flexible acrylic render is going to be far more forgiving than a traditional cement render and greatly reduces the chance of cracking. Even then, minor hairline cracks over many years are possible, but they are usually cosmetic rather than structural.
Compared to blockwork, the sleeper and fibre cement method trades ultimate longevity for ease and cost. Blockwork on a proper footing will always be more solid and stable, and is the best option if you want something that will not move or need attention for decades. The downside is the extra work involved in foundations, the higher material cost, and the need for masonry skills. For a backyard project, especially one with a bench integrated, that can be overkill unless you are already comfortable with blockwork.
Overall, if you are building it yourself and want to keep the project budget-friendly, your idea is sound, but it comes at a slight cost in terms of longevity. If you pay close attention to drainage, separation from soil and the type of render system you use, you will end up with a garden bed that looks like a rendered feature and performs well for many years.
Let me know what you think and if you have further questions, please don't hesitate to ask.
Jacob
