Any way somebody can lift the doors enough that you can attempt to turn/spin the wheels manually? Can you put some sort of (very soft, non-scratching) shim under a door and just lift it up a skosh?
If the grit is actually in the bushings/bearings of the wheels, then they’ll really need to be cleaned and then lubricated. Turning them with no weight on them should give you a pretty good idea.
Plus … it’s just fun.
Cleaning the tracks is much easier. It’s quite likely you did a fine job and that’s no longer your issue.
But “gritty” is definitely a word that would describe dirt in the wheel assembly itself.
If the wheels themselves don’t turn smoothly, then – ideally – you’d remove them and soak them in vinegar, then clean them with warm soapy water, then rinse them thoroughly, and then lubricate them – IMHO – w/food-safe silicone spray (the same kind of grease would be fine, too, but grease – though longer-lasting – is more prone to attracting and retaining dust, dirt, grime, etc.
I assume everybody knows what we’re talking about, but …
Etc,…