Finer stones make a keener edge, not a more durable one. If anything a higher polish on a soft steel deforms sooner. The burr is the problem here, not the grit.
You're leaving a wire edge. It shaves because a thin flap of steel is sticking out past the apex, and that flap folds over or tears off after a few cuts, leaving you with a rounded edge that never was sharp. Fix is to stop chasing sharpness and start removing the burr properly: after your last 6000 passes, do 3-4 edge-leading strokes per side with almost no pressure, then a few alternating strokes, then check by dragging the edge lightly across your thumbnail at 90 degrees. If it catches evenly with no soft spot, the burr is gone.
Worth adding: compound plus heavy pressure rounds the apex, which produces the same disappointment two days later. Light passes, edge trailing, maybe eight per side.
Then that section still has a burr hanging on. Work that spot alone with light alternating passes until it matches the rest, and don't move up in grit until it does.
One more: after reshaping, the new tip area will be thicker behind the edge than the rest. Thin it on the 1000 before you finish or it'll cut noticeably worse in that section and you won't know why.
Don't take the whole edge down. Losing 3mm at the tip and blending it over roughly the front 40-50mm of the blade will look almost invisible when you're done and costs you a fraction of the steel. Work on the spine side, not the edge side: you're reshaping the tip by removing metal from the top profile so the point comes back to meet the edge. Go slowly on the 220, dunk the tip in water every 20 seconds because thin steel at the tip overheats fast, and stop before you think you're done because it's easy to overshoot.
It usually is. Nobody grinds a 30 pound knife to a consistent bevel down the whole length.
Marker coming off the shoulder and not the edge means you aren't reaching the apex there, which is almost always a thicker bevel near the heel from the factory grind. You have two options: raise your angle a couple of degrees just for that section until you get a burr, then blend it back, or commit to thinning the shoulder on a coarse stone. On a cheap santoku I'd do the first one, it takes ten minutes instead of an hour.
At 60 with no storage I'd buy one splash-and-go combination stone around 1000/3000 rather than a diamond plate. Splash-and-go needs a 30 second spritz, not a soak, so the sink constraint disappears. Diamond plates are brilliant for flattening and for very hard steels, but on soft stainless they load up and the feedback really is dead, which matters a lot when you're learning to feel a burr.
Fair, and the no-dishing point is real. I'd still push a beginner toward feel, but I've seen more people quit because their stone was a hassle than because it was a plate.
It also dishes where you sharpen, which is why the middle goes first. Working across the whole surface rather than the comfortable centre patch slows it down noticeably. Doesn't stop it, but you'll get more sessions per flatten.