i need some rendering advice

hi all
got a small concrete block wall that needs re-rendering (old render looks like it was removed years ago) it between 2 windows its only around 1.5 sq mts (1.3 mts X 1.25mts)
it needs 30mm to bring it out level with the brick work either side of the windows
here is my thoughts/plan
1 basecoat of 15mm concrete (4 sand 1 cement) scarifer the surface
fit 15mm pvc beading
1 coat 15mm tarmac one coat render

or should i go with more thinner coats of concrete/tarmac one coat ?
I will need to build up the level so i can fit on the Pvc beading (10mm or 15 mm beading)

I'd be tempted to screw on strips of 18mm ply, then fit 12mm rendaboard.

would that hold up to the weather ?
i get a lot of wind and rain that side of house

just had a re-measure area is 147cm x 136 cm
so would need a joint in the renderboard

if i put up strips of marine ply, should i put insulation boards to fill the gap ?
should i screw or glue the renderboard on to the marine ply ?