Large block reads are just a readahead scheme where you prefetch the next N small blocks. So you are just stating that contiguous readahead is close enough to arbitrary readahead especially if you tune your data structure appropriately to optimize for larger regions of locality.

Well I mean yes, you can use io_uring to read the 128KB blocks as 8 4KB blocks, but that's a very roundabout way of doing it that doesn't significantly improve your performance since with either method, the operation time is more or less the same. If a 128 KB read takes roughly the same time as a 4K read, 8 parallel 4K reads isn't going to be faster with io_uring.

Also, an index with larger block sizes is not equivalent to a structure with smaller block sizes with readahead. The index structure is not the same since having larger coherent blocks gives you better precision in your indexing structure for the same number of total forward pointers, as there's no need to index within each 128 KB block, the forward pointer resolution that would have gone to distinguishing between 4K blocks can instead help you rapidly find the next relevant 128 KB block.