Best Materials and Techniques for Building Raised Garden Beds

Hey everyone! I hope it's okay to ask about raised garden beds here. I recently moved into a house with several large raised beds, but unfortunately, the wood is starting to rot, and screws are pulling through. The beds are about 2x4 high and shaped like big Ls without any bracing.

I'm planning to build larger, more durable beds in the future and have a few questions about construction.

First, what type of fasteners do you recommend? I currently have deck screws, but I'm wondering if construction screws would be a better choice, and why I don't often see bolts, washers, and nuts used—are they typically too expensive? Would they help stop screws from pulling through?

Also, is there a rule of thumb for bracing beds? My existing ones have no bracing, which has caused the wood to bulge over time. I've tried some temporary fixes with metal braces on the top and bottom 2x4s. Are those effective, or should I install a 2x4 horizontally and attach it to each side of the bed? Is there a guideline for how many braces I should use per length?

Lastly, if you have any other tips or tricks for making raised beds last longer, I'd love to hear them! I live in Ontario, Canada, where we experience cold winters and hot summers. Does longevity depend significantly on the wood quality, as well? For example, would I need heavier-duty fasteners for rot-resistant wood compared to cheaper wood?

Thanks a lot!