There was a practice of using MergerFS/OverlayFS for pooling multiple drives (often by SnapRAID users), but what's still missing (to my knowledge) is some sort of a balancing layer, that could distribute writes.

I got this idea many years ago, when first personal cloud storages appeared and offered some limited space for free. I thought it would be nice if I could pool them and fill them taking their different capacities into account. And if I could also stripe and EC them for higher availability...

I still wonder if there's something that can do this and if there isn't I would like to know why, since it looks like a useful and obvious idea.