Advice re: spacing and frequency for horizontal deadman posts in retaining wall

I'm building a post and plank retaining wall retaining wall to shore up an existing slope and establish some parking spaces in front. The wall will be about 12.5m wide by 1.5m tall, with 3m tall 6" square beams in concrete spaced at 1.25m. I know that because the wall is just over 5' I need to provide some horizontal reinforcement, and I'm planning to use horizontal deadman beams. What I'm not sure of is the proper spacing of those deadman beams. If I fix 6x6" dead-man beams of 3m long to the top four flights of the wall at 8' intervals (every second post) staggered across the top four courses and secure these with rebar or horizontal reinforcements is that sufficient?

As unhelpful as this comment is, the generally accepted advice is that a retaining wall over 1m should have SE involvement due to the risks of a tall retaining wall collapsing

Aye, I was aware of this, and had discussed with an SE - which is why I'd landed on french drain and deadman beams. But was hoping to get some sense from others who'd done this before what sorts of configurations had worked in actual practice...

Is this choice of deadman beams just a bit unusual? I'd expected it would have been a bit more common.