Re :In any case, even with a 4 1/2” brick wall there is no flexibility as there is with wooden beds.
Actually, there is if you don't use mortar. Just loose lay in a decent bond and they work fine. My mother created a winding 3ft high retaining wall at a slight leaning angle for one of her garden beds.. as a frail pensioner, who just laid a few bricks at a time as and when she felt well enough.
Most domestic poly tunnel;s won't be big enough but outside, and in larger enclosures, you can also lay them with 2 inch gaps lattice style to help fresh manure rot down etc etc, and provide heat for the seed trays above. Then remove the bricks and dig it out to put across other areas. Or fill with soil and plant directly.
My winter leeks have now flowered and I have lots of fantastic allium balls rising above their bed. Less than 6" of soil on cardboard, over original lawn and local clay.