I took a few shortcuts in my explanation and may have implied that the disk full case was similar to what was shown here (it's not completely related). All in all this is really controller dependant. Some controllers do the bare minimum to show good speeds in benchmarks, which means they never do much consolidating once every cell has been written once. This used to be the norm when SSDs were relatively new. 3 years ago this wasn't the case anymore, but for the most bargain brands (of controllers, you may be disappointed by "good" brands using poor controllers, Intel for example).

At 25% capacity you usually don't see any degrading in performance on the best controllers (I would put Samsung in there). QLC makes things harder on controllers (4 bits) but unless you are in a terrible spot (say you just filled 80%, and try to fill more without leaving enough time to consolidate), you are rarely doing terrible.

Which QLC drive did you have a bad experience with ? Samsung's QLC drive as far as I know recover their "SLC" cache pretty much always for example, according to Toms in 5 minutes of idle time [1]. And this is with their cheapest (I think) model out there.

[1] : https://www.tomshardware.com/reviews/samsung-870-qvo-sata-ss...