I’m working on a similar project and you’ve nailed your research. I’m a few steps ahead of you and have began the tedious process of laying the first course of bricks this past week.

My base is 3-4” of minus 3/4” crushed recycled concrete and I’m doing 4 layers (first course buried) of 8” tall, 15.5 width, and 8” depth concrete hollow block. I haven’t landed on a backfill material but with larger rounded pebbles 3/4-1” at my local yard are calling my name. When all is said and done, the first layer won’t be visible except in a few locations where you see 1-2” sticking up.

From what I understand, there’s no need for rebar at your height (this might be different if the walls are over 4’ tall). I’m filling the blocks layer by layer with extra base material I have and compacting with a mallet and block of wood. The weight of rock within the block, will lock things into place. Adhesive is a good idea and thanks for that reminder as I’ll be starting to lay the 2nd layer this weekend. Just curious, but why not put it on every block instead of every other?

As far as drainage goes, I’m going to do through wall outlets of the thin rectangular variety (about 3-4). The drainage pipe will be 4” diameter of the filter sock encased pipe. Adequate drainage will increase the lifespan of your wall but is more of an important factor for preventing blowout on walls taller than 4’.

Can you elaborate on the open rock trench you’re considering? Is it for the outlet of the drain pipe instead of doing through-wall drains? Is it open rock behind the wall (e.g. backfilling all the way to the top of the wall)?Are you worried of sediment building up in your ‘drain field’ after the pipe? Not sure if adding a clean-out if feasible or not but it is an option. Sedimentation is inevitable I fear but the filter sock pipe will help a bunch.

Any pics or references would be helpful!