Retaining Wall Issue

If you're up to using CC, It's fairly easy to create a fence without a texture, so it is -in fact- invisible.

If you're not, you're stuck with either a (half wall) or a fence on top of your 'retaining wall' or just using a half wall itself. A half wall is a bit more narrow and does not have foundation designs, but it is very much just a retaining wall :) Another option would be to use the debug walls that are manually placeable and come in some stone and brick designs.

I have a method (uses TOOL mod) to get rid of the wall part if you would use a foundation with a full wall. Link to video
This works for any continuous full wall, not just roof walls. It doesn't work on half walls, since it would leave the half wall trim. It will only leave "phantom wall pieces" on side ends. So in your case, the parts where you'd want a gate of some sorts or want to connect it to something else like a hedge. Your choice to use it or not :)

I've already solved this one. Apparently, I missed a built in fence that worked perfectly for it. You can see it in the image below, as it runs across the front of the lot.

https://i.imgur.com/F3mZafa.png