Do It Yourself

So I'm getting ready to build a patio with a flower box on 2 sides. Please let me know if there's a better community for this question. First I'll talk about the patio.

I think I need to have a wood border to the patio to make sure it all stays together, especially because where it's going has a minor slope. I'm hoping to use flagstone and I would like the spaces between the stones to be filled with moss. Where I'm digging up already grows moss naturally so I'm hoping to just save that moss and have the top layer before the stones be dirt and use that moss to fill in the gaps.

For the flower box, I'm thinking I'm going to use some .75"x2'x4' boards, sink them a foot into the ground so they're 3' tall (mostly to keep away groundhogs), then use some 2x4s to stabilize the walls and a few bracers going across. Then fill it with dirt and start planting.

Is there anyone here with enough experience to approve my plan? Am I over engineering or is that a good idea for someone good at being sloppy and who hasn't done this before? Should I fill the bottom of the flower box with rocks or something to save money on dirt? Any changes I should make to my design? I do have a schematic I made if you think that would help make sense of my idea.

Thanks for reading!

Edit: Schematic!