prep for rendering garden walls

Morning all

Hope you are all keeping well.

We recently completed an extension build. Ours is a tricky plot so we have also had to build several garden (concrete block) walls to wrestle it into a usable space.

Our first choice is to render the walls however our builders have back-filled most of the walls without consideration for them being rendered. I am under the understanding that the walls need a moisture barrier of some sort. Most of the walls are back filled with loose rubble which i presume is better that moist soil sitting against the wall? However most of the walls will be topped with soil. So i have some questions if anyone can help?

1. I'm thinking i need to pull all the backfill out (!) to get a moisture barrier behind the wall - is this correct or could i just do the section that will have topsoil?
2. What is the best DPC/DPM to use? Need something cost effective as i probably have a lot to do! I can't use dpm paint as the back of the wall is too rough an holes in the mortar mean i can't guarantee coverage.
3. I am concerned that once i get a sheet plastic of sorts behind the wall, this will all be ripped by the rubble going back in. Can i protect this with a panel material? Correx, polystyrene, Osb? I realise any sort of wood wil rot but it make protect the initial back fill and setting? Again need something cheap.

I am also aware there should be drainage at the base so i am drilling weep holes through, however we are down to bed rock so there is lots of gaps for water to move away.

I have attached some pictures. ignore the very back wall with the horizontal battens as i am likely to convert that into a form of green wall.

Thanks so much everyone!

Does this help ?

Just realised it won’t, as your question is more membrane related than render application.

Link deleted