I haven't done the math, but my gut feeling is that the relatively modest increase in SLC cache size could not on its own lead to that big a drop in post-cache

I haven't done the math, but my gut feeling is that the relatively modest increase in SLC cache size could not on its own lead to that big a drop in post-cache sustained write speed except possibly where QLC is concerned, and especially not when mitigated by a generational increase in per-die performance. Samsung's drives are still operating with SLC caches that are smaller than the theoretical limit, which means that even after the cache is full, the drive should be able to accept some amount of writes that bypass the cache and have near native TLC performance; flushing the cache can be deferred a bit longer until the drive is actually starting to run out of free blocks, as opposed to typical QLC drives with maximally-sized SLC caches that run out of free blocks at the same point where the cache runs out.